mehdismummy- this might be e bit of a long post but wanted to share this as I sort of come from the "other side of the fence"!
I was once in the same situation as your sister. My lovely sis wanted to leave her dp (they have two children, but no violence or abuse in any way in their situation) and I really regret now that I didn't support her enough in her decision.
I work with her now-ex dp, and he and dh have got to be very good friends during the 8 yrs sis and her dp had been together, and so dh and I had some close links with him, in a similar way as I suppose your sis has with your dh through her hubby. I felt incredibly torn and handled the whole thing really badly, I could have been so much more supportive of her and the situation she was dealing with and I regret this hugely. I've apologised to her since, thankfully we are very close again now although I think it will take my sister a long time to fully forgive (if she ever can) what I can now see was me letting her down big time, even though I didn't mean to and am so ashamed that I let her go through such a difficult time in her life without my full support.
This sounds like me trying to justify it, and it so isn't but there is no handbook to refer to when a close family members' relationship breaks down, nothing to give you/her a ticklist of what you "should" do and what you "shouldn't"- I made decisions/acted in ways I thought were appropriate at the time but with the benefit of hindsight realise I seriously misjudged situations and made some really bad decisions. I was sort of feeling my way through a situation blindly and made some big mistakes, which I'll always regret because they hurt my sister. But that is about how I handled a situation when realy it was about her- and that's what I just couldn't see at the time, that whatever I felt or thought was completely irrelevant becuase she needed me to be there for her.
Like other posters have said, be strong, you sound like a lovely person and you will be able to get through this without your sisters support- she might be making bad choices now but don't write her off completely, instead concentrate on yourself and ds rather than wasting precious energy on worrying about her opinions. Hopefully she'll realise the error of her ways and turn out to be your biggest supporter (like I am now of my sister and my wonderful neice and nephew in their new life)
Good luck
xxxx