Not to be? or, Not to be?

So, my sister has been dating, and is now no engaged to a guy that is .....we will say.....creepy. Now, she, and he, have a few learning disabilities, and they have a hard time comprehending things. So as much as we (the family) know that they are not right for each other, we just cant make her understand that. They are %100 positive that they are meant for each other, but we always feel uneasy and creeped out while around him. Not only that, but they are literally joined at the hip now. Which really worries me, because I think there are a lot of control issues in the relationship. We are all worried about her, and we don't want to see her get hurt, but we have tried expressing our concern to her repeatedly, and she always takes offense, and accuses us of judging him. Are we in the wrong? are we being too hard on them? Or should we follow our instinct?