Have you tried searching this site's PRs using the "advanced search" feature ?
(see link at base of PRs search page). This brilliant facility lets you search just about anything with just the minimum of known data. I've tried searching "Baptisms" using just "Herbert", "Jo" (to find all possible spellings of John) and "An". This gave 53 records dating back to 1696. These can be easily sorted to give a table in chronological order, stating dates, names & parish, so very easy to quickly sort thro. The majority are from the Newent & Oxenhall area, the flat agricultural area northeast of the hilled Forest "proper". This is quite far & different from Newland & Bicknor which (as you probably know) are the western edge of the Forest on the River Wye boundary, more of an industrial area. A few of these hits standout as being nearer to your marriage parishes so in my mind are more likely, particularly this one:

Record ID 56507
Entry Number 33
Year 1813
Month Dec
Day 12
Parents Surname HERBERT
Child Forenames Charles
Fathers Forenames John
Mothers Forenames Hannah
Mothers Surname
Residence Forest of Dean
Occupation Collier
Officiating Minister T. D. Fosbrooke Assist[ant] Curate
Event Baptism
Memoranda son of
Notes
Register Reference P275 IN 1/7
Page Number 5
Parish Chapel Ruardean

(So the advanced search cleverly found Hannah, always worth looking for this spelling, plus Ruardean is precisely the same area as Newland/Bicknor).

I wonder if Charles is a possible relative ???

Similarly checking the "Burials", a similar trend develops with some "hits" in Ruardean again, plus neighbouring Monmouth, Redbrook, Lydbrook and Walford. These deserve studying perhaps for clues.
As do the results of searching "Marriages" for "Herbert"(both groom and then bride surname). Here check the witness's names . For this reason perhpas this "hit" is one of those deserving more attention:

Record ID 44592
Entry Number
Year 1849
Month Oct
Day 30
Grooms Surname HERBERT
Grooms Forenames Philip
Grooms Age full
Groom Condition widower
Grooms Occupation butcher
Grooms Residence Hewelsfield
Grooms Fathers Surname HERBERT
Grooms Fathers Forenames John
Grooms Fathers Occupation butcher
Brides Surname HERBERT
Brides Forenames Eliza
Brides Age full
Brides Condition spinster
Brides Occupation
Brides Residence Tintern
Brides Fathers Surname HERBERT
Brides Fathers Forenames Isaac
Brides Fathers Occupation farmer
Licence or Banns banns
Date of Banns
Signature or Mark He marks she signs
Witness 1 Edward Thomas Ball
Witness 2 Anna Herbert
Other Witnesses John Herbert
Officiating Minister
Event Marriage
Memoranda
Notes
Register Reference
Page Number
Parish Chapel Tintern Monmouth

Basically this is a very usefull searching tool indeed and may give some clues to build on, I do hope so.

Finally, have you found the girls in the Census' ?. It's possible they are sharing their married homes with their elderly parents, perhaps ?.

PS !
Also, this could help you search for possible children of Sophia & Ann, especially as it very cleverly allows for spelling variations, as well as being far quicker than trawling thro the individual PRs.

How about these for John & Ann, they stand out from the list wrt the dates (just after their marriage), the location (near to English Bicknor).

129280 1817 PARSONS Enoch, John Ann, Forest(Ruerdeanhill)[sic], Collier, Hope Mansell
12897 1824 PARSONS Thomas, John Ann, Ruardean Hill, Collier, Drybrook
13246 1829 PARSONS Joseph, John Ann, Ruardean Hill, Collier, Drybrook
13348 1830 PARSONS Isaac, John Ann, Ruardean Hill, Collier, Drybrook

HOWEVER, I can find NO baptisms for James Brain & Sophia/Sophie etc at all.
I CAN see several Ruardean baptisms for father James Brain & Hannah/Mary around this period...

Looking at the 1841 census for Ruardean Hill, (and knowing some families lived so close together) - there are lots of PARSONS/BRAINS virtually next door neighbours

with a Martha HERBERT * age 65 and not too far away, James TIPPINS, Sophia BRAIN 40 and then Ephraim PARSONS 25 etc etc.

FOD records, burial at Drybrook 3 February 1846, Martha HERBERT age 72 years (Ruardean Hill).

"I would like to find the Herbert girls parents and any other siblings and if very lucky, descendants of the two girls."

Hi Jean,
I realise I was unable to find the parents, I wish I could have done. However I don't have the benefit of many census databases to refer to except what I've shared. Please understand I was just trying to build an "image" of the overall family group, which may have quite possibly led to the girl's parents albeit at a tangent as twere, perhaps from another PR. It certainly showed, I felt, an image of likely family names and so on which would be usefull when discovering witnesses at Marriages for example.
I was also unsure how well you knew the Forest geography; you stated Newlands rather than Newland and of course this may have just been a small typo error, and there is a famous South African sports stadium called that, however it's the first time I've heard/seen it wrt this forum or the Forest generally. This plus the next post made me feel you may be thinking of the Oxenhall area, where so many Herberts reside, however as Slowhands has also mentioned this seemed highly unlikely to me. In fact as other's have mentioned it seemed to me likely there's an earlier Monmouthshire link as is often the case with the Ruardean neck of the woods.

However please recall your original post actually stated "I would like to find.......any other siblings and if very lucky, descendants of the two girls."

I also feel that sometimes we have to "speculate to accumulate"...which I feel I've done. I believe this has proven effective as I think I found many descendants and a possible sibling of the two girls. I think Slowhands has confirmed these findings. So maybe you have been lucky ?. Re the rest, by showing my research I believe I've helped other researchers if only by saving them wasted work.

Good luck with your continued efforts, I look forward to seeing the final outcome.

Regards Jeff

Thank you Slowhands. This is the correct couple. I have the grooms parents and am working on the Herbert side. The only other possible reference I have found is a baptism (from this site) of a Sarah, Parents John Herbert, mother Ann, living in Norton. Child Sarah baptised in Norton chapel. 13 May 1798. This may be another child of the couple who I believe to be the parents of Ann (you found the same baptism as I did) I cannot find any reference to Sophia either but thank you all for searching. I will search to see if there are any records of the marriage of John and Ann. It was interesting to note that in the memorandum of the probable baptism of Ann it shows the words "John Herbert junior". May this indicate that Johns father was also called John Herbert?

Could this be Ann's burial at Drybrook?

Record_ID: 78992
Entry_Number: 1395
Year: 1883
Month: Oct
Day: 18
Surname: PARSONS
Forenames: Ann
Residence: Bilson Woodside
Age_at_death: 90
Officiating_Minister: J H Hawkins Curate
Event: Burial
Cause_of_death: [not sta
Memoranda: Name is annotated OY
Notes:
Register_Reference: P109 IN 1/12
Page_No: 175
Parish_Chapel: Drybrook

However there doesn't seem to be an Ann Parsons born in the 1790s in the Forrest on the census after 1851

Husband John died in 1853:

Record_ID: 70042
Entry_Number: 970
Year: 1853
Month: Feb
Day: 18
Surname: PARSONS
Forenames: John
Residence: Ruardean Hill
Age_at_death: 56 Years
Officiating_Minister: H.G Nicholls Incumbent
Event: Burial
Cause_of_death:
Memoranda:
Notes: Note in Name Column-O.Y.
Register_Reference: P109 IN 1/11
Page_No: 122
Parish_Chapel: Drybrook
Soundex: P625