Personally, I think it would be daft, if not slightly frowned upon, to loan a tiara from your MIL’s family when you’re marrying into the RF, who have some of the most spectacular jewels in the world. Imagine the column inches generated from Meghan not wearing something from the Royal vault, forced to ask Earl Spencer to wear their family tiara etc etc
or look at it another way, wearing your husband's Mum's tiara when she has passed away might be a nice thing to do.