I really think, speaking with my professional hat on, that you are going in with the right attitude. While your *suspicions* may be right, there's nothing to be gained from rushing in and doing an "intervention," that might very well end up ruining your niece's wedding. You want to observe the whole situation in person. And although it may be just as you suspect, there may not be much you can do. If your sister was mentally incapacitated to the point of not being able to take care of herself (i.e. hygiene, daily tasks, safety) or there was a question of some kind of abuse in the home (to her, or to a child or elder), or substance abuse, you might be able to force the issue. However, people make poor choices like this all of the time and there's not really much you can do to stop it, except to tell the person how you feel, recommend therapy, and let her know that you are there for her. It sounds like she had issues before, that might not be easily solved, and if I were you, I would focus on making sure your niece has a nice wedding!
And, just to re-iterate something I've said before, I met my DH on 6/27, we moved in together on 8/1, got engaged on 8/28, and married on 12/8. Thirty four and a half years later, we're still married and happy. DH's family tried to tell him there was something evil about me, something so bad, that he would eventually find out, or I would leave him, because I was from a "high class" family. So, you never know.