Avery Wake up, this is a completely different culture. We have at least one person here that is over 30 and has been locked up by parents for more than ten years. Do you want to editorialize on that? I'll bet they have a happy life.
Yes, it is another culture. Nevertheless if you do something do it right.
From @ZeynepFox posting her future husband will lead their relation including sometime spanking for her for her development.
They live in Germany and they respect and following the German law.
As an adult you make your own decision and nobody has the right to do so for you without your consent.
If, as a young adult, you are aware that you do not know everything and that you also have less experience, this shows maturity. Therefore, when it comes to important questions, it is good to get opinions and advice from others who know more or have more experience. One way is to ask your parents. In the end, however, you decide for yourself and stand by it.
If her mother-in-law insist to have the key at her wedding and to keep it against his will than he shows he is not the leader of his own family. Apart from that, it is against the law to use $240 StGB (coercion).
If he agrees than his mother should ask herself what she did wrong with her education of her son that he is not able to fulfil his duty or can't handle his wife.