Once the twins are born, you can use SimPE to change the family tree but you can't change the genetics. (They probably look better if Peter is not their dad, though -- he makes UGLY babies!). If you can't use SimPE, you're just out of luck, I'm afraid.
Yes, I'm afraid I've never used SimePE and I'm too scared to try it. What is the worst thing that could happen whilst using it? Is it possible to actually make your games unreadable or something equally terrible? If not, I might consider it. :p
Couldn't they use Insim? You can remove family ties with Insim and then reset them to the right family member. Easy enough for those who are too terrified of Simpe, there is very little they could actually get confused about, especially if they use Insim object edition because then they could just use the relationship object and would only be fooling around with the options related to the relationships.
Insiminator is very easy to use, is it? Is there anything I could do on it that would potentially annihilate my game? Thank you, I'll see about that.