He was probably cheating, had a few weeks “not talking” and that’s when she got pregnant.
it’s true that mixed kids can look different from each other. However, in the majority of cases most do have some sort of resemblance to their sibling.
Isla clearly looks like she could have a different dad, explains why less is spent on her.
Also, doesn’t Paddy already have lots of kids? So I don’t think he’s one to put up a big drama over it.
I know some people are from cultures where blended families and everyone having kids with different people is normal.