in real life, children can also be different - you’re right, in general. but their common features will still be easily visible. but if the children are VERY VERY different, then adultery is more than likely. Yes, there may be an old ancestor who “suddenly shot out of the past”. this can happen - but this is revealed by looking at a family photo album (I hope most of us have such an artifact in our families, right?) my aunt had three sons. the eldest looked very much like a gypsy. so then the aunt’s adultery was discovered, and the other two sons are very similar to each other, although there is a temporary gap of 10+ years between them... that is, if there was no adultery, then there will be children - give or take - but in general they will be recognizably similar to each other - it will be possible to say that they are siblings. not identical (like twins) but similar. my brother has two daughters - they are not very similar to each other. but they look like family. for example, she looks like a grandmother on her father’s side and the other looks like her grandmother on her mother’s side. that is, there will be similarities with family members. and there shouldn’t be any completely different children. children who are completely different is suspicious.
So let's go back to Becca and Lilith - they are very different. True, Lilith has something in common with Shannon.
It’s a pity i do not remember a frame in which you can compare MC and Becca.