This is the worst bit. I promise. It will get better.
Your MIL and SIL are not your friends, any more than he is. From now on you back yourself and your son. You're in shock, I would be tempted to go to the GP and ask for some diazepam so you can get a bit of rest, which will enable you to take some action. Your focus at the moment is just to get yourself back on a bit more of an even keel, do whatever you need to do for that to happen. Treat yourself as though you're dealing with grief.
I know you have recently moved to that new area, do you have any friends or family that could come and visit you, or you them? You could do with some support, can you think of anyone you could call?
Your son is 18, and his relationship with his dad is something you can be supportive of, but you're not responsible for it any more. You don't need to feel guilty.
I would also say that he has someone else, he probably feels ashamed of himself which is why he can't face you or his child, just instil in your DS that this isn't personal, dads' kind of lost the plot and is making some bad decisions at the moment, but it will be ok, he's got you.
Hang in there, just do the day you're in, and if that's too much just do the next hour, or half hour. Break it down, do the next right thing, try to eat. Order a food delivery with all your favourites, some ready meals to chuck in the oven, whatever you fancy is better than nothing. Try and get out of the house once a day, scrape your hair back, stick your dark glasses on and some lipstick and hold your head high.
In practical terms yes you will be entitled to half the house, make an appointment to see a solicitor as soon as possible.
It will get better, you will be happy again, possibly happier than you have ever been. Life will be good again, don't despair.