People change their faith connections. The CC certainly works at converting Protestants (witness ‘Journey Home’ on EWTN). I understand that Protestant churches around here have many ex-Catholics as active members now. At least three pastors here were ex-Catholic, one a former Catholic priest. One of the priests in the area was raised Protestant. They convert for all sorts of reasons, including marriage, disaffection because of some rude priest, rejection of certain key teachings or practices, etc.
Good family relations are important, and we shouldn'r let differences between, say, Presbyterianism and Catholicism cause family ruptures. That was the pre-Vatican view, when it was a big 'sin' to even enter a Protestant church. This sort of policy seemed to be based on bitterness and bigotry, a clear contradiction to the message of Christ. Remember how he chose a Samaritan, a man of a different faith, as the hero in one of his most beloved parables? That should tell us something.