We have had a few run ins with DW's relatives over stuff like this. We explain our reasons. After a while they get the message.
What is funny, is that after taking care of him for a day, some of the more vocal ones have stopped making comments about what we should do. LOL.
Mr W's pediatrician has been great. She came right out and told us we had to do thing differently.
Same here. My mother-in-law openly scoffed for a while, because she wasn't around him that much early on, and I think that a lot of it was unfortunately motivated out of emotional loyalty to her other grandchildren. No longer.