If her inability to get into a relationship persists then I think there are two or three areas to work on.
1 Either her character is not welcoming enough to men because no man will want to see a troublesome woman and will still want to go ahead to marry such else the marriage will be a den of fighters instead of lovers.
2 It might be spiritually related or inclined. I have heard of cases of people being caged spiritually, and a beautiful face of a lady will be made to look like a man's face to men who may be interested in her. So I think prayers will be a solution to her if she falls in this category.
3 Let her be happy for her sister and wish her. Good things will or might never come our way if we don't appreciate the good that happens to other people.

That's my little view and contribution.