Thanks for your sincerity, @swedoise. It´s hard to say but maybe you cannot do anything. The only thing you can do is: be there, whenever she needs you. It´s a known pattern. I´ve lived it with a very good friend. Some people (often due to happenings in their own childhood) "need" to be under so.´s spell. There are several levels. Some of them just need having a partner who gives a direction, others need to be completely controlled and then there are people who even resist physical threat in a relationship believing it´s an intense form of love.
Usually the only way out is an extraordinarily "extreme" (not neccesarily violent) moment. But they´d have to experience it on their own. There is no chance to convince them just talking to them. Maybe they understand your point of view or even comprehend that they are doing a mistake. But they "can´t" leave it. They would always relaps into the same pattern.
Show her love and that you are there whatever happens. The closer her relationship with the family, the better your chances to get her out some day. Good luck! :-)