If you’re still in the planning process, the best way to handle this is to hear them out calmly, BUT in the end make your own decisions. Keep in mind, if it’s a small battle, letting your MIL pick the song the parents come down the aisle to is small potatoes. Know when to hold your ground and when to appease people. In the case of family, you will be seeing these people long after that tantrum about not wanting a blue pin for the sign-in table. Pick your battles. And, always to do so respectfully.
If it’s the wedding day, do what you can to create a buffer. I’ve told a few of my clients, “Let me know if I need to come pull someone out of the room.” I’ll be the bad guy if it means the bride gets to have a wedding not filled with drama. My heart breaks when I hear stories of outrageous family problems on the wedding day. Surround yourself with people you love and people that can keep you calm.