It seems like your sister is recapitulating the original trauma in the family with you and your siblings. She is alternating between angry & condescending (like your mother) or wounded and emotional (like your father.) The short version here is that she seems to be doing to you what was done to her. Of course these are just guesses on my part, but this would be one way to explain what is happening.
What you need is a family therapist. Along these lines I have two suggestions. First, I would round-up your other siblings and find a good family therapist in your area. This site may help you find one — or you may find one by looking at the “find help” tab at the top of the page.
Once you and your other siblings have agreed upon the therapist ask your sister if she wants to join in. It is important that she be asked, but whether or not she decides to go — the rest of you go for the therapy.
There are other ways, of course, and my second suggestion is to consult a family therapist on your own and be guided by his or her direction in the matter. This path is the safest and most direct help for you because you will get another expert opinion on the matter before bringing your siblings in.
However, now is the time to get some help. As your sister becomes a mother, the family dynamics are likely to be activated at a deeper level. The initial goal is to help yourself first — so you can best learn how to be effective in your relationship with her.