Why do in-laws not let go?
An answer can be found in the Ramban’s commentary on the passuk in Beraisheet, where it says that a man shall leave his parents and cling to his wife. A son/daughter will cling to their other half, and the two halves make one unit together. They will cling to someone who is really part of them. In that respect, they are related to each other than their parents. That makes some parents rather unhappy.