That would tell you the parents of both girls.
But OP was wanting to know wnich of these girls was the one that married Isaac and thus was her ancestor.
The fact that the one that married does not name a father on the marriage cert suggests she was illegitimate. Therefore , if the birth certs of the girls comfirm one to be illegitimate, then that looks the most likely candidate. The censuses are imdeed suggesting that onegirl may indeed be the illegitimate child of Elizabeth and the other a child of John (James in 1851) , rather than one a child of Braithwaite and one a child of John.
If however the birth certs of the girls confirm the latter (as OP was thinking) then they will not clarify which one married Isaac but failed to name their father on the cert!
Im of the opinion one IS the illegit child of Elizabeth (as censuses suggest) and thus it is that one that married Isaac, but certs needed to confirm 100%