Hi Poppy 1992,
I was on the forum and saw that your post was unanswered. I am sorry to hear of your mother-in-law's and niece's current situation. It is good they have you and your partner to help them.
It does sound as though they are both really struggling with their separate issues. With regards to the TLC of their house, could they get help with this from social work if you gave them a call? Or maybe there would be some kind of grant/funding they could claim to do their house up? It maybe worthwhile speaking to your local Citizens Advice Bureau about this if social work can't help.
With regards to their own mental health, they do need to speak to their GP about this. If they won't go along, can you or your partner maybe either go along with them or speak to the GP on their behalf? Although this will depend upon if their GP will discuss them with you. Social services may also be able to help in involving the GP and CMHT.
I'm sorry I can't be of much help, but others on here maybe able to suggest something too. I hope things improve for them both anyway. Best wishes, Aileen.