DH has an extremely toxic younger sister who picks arguments every time we speak to her. She lives with her mum, has now taken over many rooms in the house which she fills with junk, even drills random holes in the walls and has so much junk in her room that she now shares a bed with MIL. She is 26 and she does have aspergers/adhd but her mum refuses to seek treatment.
It was because of her condition plus the fact that MIL allowing us to live for 3 years in her home (allowed us to buy a London flat in our 20s) which made us feel like we have to repay MIL by being v nice to her. Tbh she is very needy and glues herself to MIL, every time we call MIL or even invite MIL, MIL allows her to tag along or puts her on speaker phone. Even if SIL makes her late by an hour, MIL would allow to tag along. When the other SIL visited London with her fiance and DH and I took leave to take her to Brighton, the other SIL also copied her mum's parenting technique and took her to Brighton and the first thing she did was demand to be taken to the shopping centre to buy a swimming costume even though she had 3 swimming costumes offered to her which kinda ruined the morning as obviously the whole group of 5 people had to tag along or it would be seen as 'abandonment'. My DH suggested leaving the 26 year old at the shopping centre to sort it out while we went to the beach and his other sister screamed at him for being abusive/unkind even though we all agreed that the swimming costume debacle was ridiculous, just that it is apparently the family policy to pander to this SIL. Not because of the aspergers which no one even acknowledges but just because.
Tbh SiL was quite unbearable when we lived with her and would pick fights with us at the dinner table. Thankfully she was younger then (younger than 18) and I used to 'bribe' her by buying her coca cola and treats for a peaceful life.Unfortunately she is older now and earns a bit of spare cash writing online which is great for her but means my bribes of buns from gails bakery and Rose milkshakes don't have any effect anymore, she now wants flights to southeast Asia and drops hints from time to time (we did once pay for her to fly to my home country in southeast asia for my wedding as we wanted MIL there and MIL couldn't afford it and it was another 2 for 1 offer).
I have also come to the realisation that it is MIL we are grateful to, not her. I used to encourage DH to be kind to her as I felt that it was good for DH to be close to his family.. Tbh DH's sisters have both moved abroad and barely see her or MIL, we see them a lot as we all live in London. Also I think it's bad for DH's mental health and he admitted that (he is under a lot of work stress and is in therapy for that which is paid for by his company health insurance so recognised as an essential). While we will always he grateful to MIL, I am not sure it should extend to MIL's frankly weird way of attachment parenting a 26 year old and if she wants to do that, that is not really our business but that doesn't mean we need to be shouted at over the dinner table or phone on a weekly basis. In any case I have tried my best to be nice to her for the past 9 years I have been married to DH and frankly perhaps there are limits.
I think the best thing is to go LC or NC but not really sure how to do that. I think dh would still benefit from a good relationship with his mum even though it can be a toxic dynamic but as for the SIL I am less sure. In fact us going NC may also allow MIL to realise that she is basically low contact with 3 of her children and that can't just be a coincidence. The other 2 moved away as soon as they could and barely come home. They have done this despite having no financial plan which is why the other 2 SILs are now living with their inlaws as they cannot afford to rent/had to move to their DH's home countries as a result despite being 29 and 32 respectively.
I probably would get a lot of flak about this because of the aspergers and in the past mumsnetters have advised me to be kind to her and help her be less isolated. However whatever I can isn't helping and the crux is her mum refuses to get her therapy or mental health support. That is something I or DH will never be able to change.