In this case I think it's like 99% that it was a continuity error. 4 hatched in 1, 7 hatched in 2, so that gives us 11 swimmers. The 1% chance that is not a continuity error is based on things we can't completely rule out, like Mr. and Mrs. Swimmer having 2 more babies after the second movie.
I must add to the above, that I interpret the word "twin" in a liberal way in LBT, because "aunt" is also used liberally in LBT. I think this because Cera, despite being Dinah and Dana's "aunt", calls them her "cousins", which implies a more distant relationship. If she were really a sister of Dinah and Dana's father or mother she would call them "nephew" and "niece".
However, if you take the word "twin" to mean litterally twin, then the chance that it is a continuity error are 100%. There can be no other explanation.
Hope this will you give some peace.