I've wondered if the fact that their dad never says no and was never the disciplinarian and instead was the good time charlie buying their affection over the years if that plays into their decisions. That's a heck of a lot more comfortable than a mom who makes you do your chores and homework.
__________________
Of Course!!!! Come on you know why they are there. He is probably more permissive, and more generous than before. Plus he is drunk and that means they can get away with a lot more. That's where I would want to camp.
Its not about you. Perhaps back in the day they were much younger - or maybe it was just the fighting they didn't want to be around. He also lives in "their"house. If you had stayed in the house they might have stayed with you. Its a lot for a teenager to move and I believe you said your house now is much smaller.
Kids aren't the most empathetic in fact they are very egocentric. it takes being an adult and going through some crap yourself before you can look back and say "Should have been there for my parent instead of wrapped up in myself"