Hi @ConcernedSister Might I ask if you have ever talked to your sister and listened, I mean really listened to her? You can listen to her without even asking questions too, just by taking notice of what she enjoys and doesn't enjoy, lots of things really.
The world is seen in different ways by each of us, depending on what has happened to us or what we've seen happen to others. It's complicated. Your experience of the world, even family life is going to be different from your sisters' experience, even if you think you know all about her, you won't, like you have your secrets, she will too.
What I'm trying to say is that making judgements about someone's behaviour because you want them to be a particular way might not be the most effective way of approaching things.
Of course, I don't know the details, and I don't want to either.
Do you think your sister would be happier if she did things differently, behaved in more acceptable ways and thus she wouldn't have so much trouble for herself, or be so much trouble for her family?
I'd suggest it might be helpful to your sister for you to approach the issue from her perspective rather than from what might be your wish to have her be a different way.
I'd really be careful of approaching her husband if you already have a fragile relationship with her, as it could be seen as an attempt to remove what support she does have.
Have you tried just being there for her, without feeling like you have to 'fix' either her or the situation?
Simple, non-judgemental support can be the most effective help for someone trying to be a better person in any way at all.
It's extremely easy for people to look at something like the DSM and over diagnose. After multiple times of actively listening to your sister and observing your own wants and wishes as regards her, if you really think there's a problem, it would be best to refer her to a health care professional rather than make any assumptions for yourself.