@BeetleBee99 I am truly sorry you've had that situation forced on you you. It sounds like the frying pan has caught fire and the devil is about to sling it in the deep blue sea then jump in after it!
The only advice I can give you is neither cute 'n' fluffy nor particularly plateable, so I'll just get right on and say it:
You need to call social services TODAY. This morning. Actually NOW. I know it feels like the hardest thing in the world, but you know what's even harder? Staying in limbo for the next three years then trying to deal with it - especially when the phone call to child services goes "Oh no, how terrible! And it's been like this for... 3 YEARS? Ok hun, well if you've made it this far I'll check back in two decades to see if anything's changed. Don't worry though, I'm posting you a gold star for effort and a copy of our pamphlet "10 reasons why it pays to be a winner but sucks to be you :D!"
It's cruel to be kind time, only thing that matters is what's long term best for those kids, not how you or anyone else "feels".
You're not in a position to full time raise those kids.
Their mum's going through whatever, and for the foreseeable future needs to be kept separate from them for everyone's safety.
Their dad doesn't sound like he's able to give them full time care.
Child services can and will move heaven and earth to provide the right support for those kids, you'd be amazed by what they can make happen and the budget they have to facilitate it.
And even if they go for the measure of absolute last resort, even if they go with the nuclear option of giving the little ones an alternative place to live whilst everyone works on a long term solution - how bad is that, really?
You'll get phone calls, visits, access and such just the same as you would if they lived with one or both parents
Dad will likely get some sort of arrangement same as if the little ones lived with their mum.
They can offer mum all kinds of support and assistance to help her with her current situation, not just "well get rid of the bloke then we'll talk", I mean long term psychological support to try to find the root causes of this harmful behaviour and deal with those, then start to think about involving her with the little ones again in time.
Most of all though, the kiddies with be with people who are really really well equipped, set up, prepared, able, have all the time, energy and desire in the world to give the kiddies a temporary home that's happy, safe and nurturing.
You've got to do what's right for the kids really.