I'm going to suggest you read this thread,
because it really opened my eyes to the need to maintain independence in life and that my boyfriend and I (after 6 years) still aren't actually ready to move in with each other.
I've been in long distance relationships where it seems like the simple solution is one of you has a house, one of you isn't super tied down, so just move and be together!
If you move away from the place and people you know and allow yourself to be dependent on his existing life, you risk feeling trapped there no matter what ends up happening with the relationships within that house.
The whole story wasn't very clear. You said she is his ex but that they still live together with their children and they are now in a relationship again. Can you explain what happened there and your relationship with her? I also wasn't clear if you've visited them before. You need to do that. No amount of seeing their life on cam can really prepare you for being in the middle of it.
You also need to spend some time having some real detailed conversations about everyones expectations in living together. This was what really hit home for me from that thread. We've never talked about the day to day details and that is a recipe for a real surprise once all your stuff is there and it's not such an easy situation to reverse.