If they are shutting her out now, imagine how bad it's going to get in the future? And if the father can't stand up to them, he's not going to be much help (no matter how loving a father he is). She's setting herself and them up for a miserable future. If they're not married, then the father has no legal rights over the children (sorry, but that's just how it is). A move is always difficult for children but they are far more resilient than we give them credit for (I know - I moved twice before I was 11) and it will be upsetting for them as they come to realise how their father's family is treating their Mum. Sometimes, as a parent, you have to make difficult decisions on the part of children - they'll miss his family to begin with but I'm betting that, within 6 months, they'll have made lots of new friends and their Mum will be happier too (always a bonus!)