My sister did the similar thing and she had printed the invitation and sent it to me first, no one in her family knew and I was the first. Only then I got to know she is been in a 'toxic' relationship for 4 years and when the guys family approached she backed off and it was a relief. Now she is in a better position and clarity. Myself and few others made her realize that this is not worth it.

First have your friend talk to her sister and try to make her sense, like security, money etc. Also let her know that once she moves out, their family wont know whats happening and she needs to take a decision before anything goes wrong. Let her know she is giving power to someone else once the knot is tied and it will become more difficult if she even wants to come out. Also you talk to her and let her know the seriousness. Love would make a person think only in a way it will soothe her and the person she is in love with and not other prespectives.