Its not her fault and the twins are aware of that, but its hard to be around someone who is family of someone who hurt you mother
But still, the mother lied so
I don't think it's being with Vanessa herself that they wouldn't be able to do ever again, but being with her while under the uncle's roof. Vanessa still lives under the uncle's roof, so visiting her is "visiting him". The brother would need more time since he's weirded out at finding out he liked his own cousin romantically.
Yes theres also that part
How is that vanessa's fault. It's her uncle not her father. And even the twins know how much of trash the uncle is even to vanessa. It's just dumb. If it was her father then i could agree with them. But uncle nah （￣へ￣）