Sounds like she's going through both depression and a MLC as well. I don't think you can handle this by yourself though, she badly needs help. I really think that you need to speak to her mum about her, especially since she's abandoned the children.
Do you live in the US? I can help you more if I know where you are. Over here you can get in touch with a crisis team that will assess her and can tell if she needs help or not. Before you do that though, have a quiet word with her mum. Your W doesn't need to know especially if you mention to her mum to keep it to herself. I think on this occasion she will do smile Be careful about any other issues you talk about with your MIL now, don't forget she's her mum and will be on her side now. I know as I used to get on well with my MIL, but can't talk to her now about anything relating to me and H.
I know my H refuses to get help for his depression, but I don't feel he is bad enough for professionals to get involved. I havc mentioned anti-ds to him before and counselling and he doesn't want to know.
Keep on doing what you're doing to follow the DB rules. You'll get there smile

H47 me48
T22 M21
S20 - Got high functioning autism
3/2000 H admits to OW and moves out to live with her
11/2000 H moves back home
2/4/13 H moves out
H tells me he wants D 6/13, but now he can't afford to!