Do I add my soon to be MIL to my close friends on Facebook?

I understand that you don’t have to be friends with your SO’s parental figures, but my soon to be MIL and I are friends, she’s probably my 2nd closest friend other than my fiancé. I make my friends list private so only me and them can see, my fiancé is on my list but because of the friends list settings only he can see, not her and his siblings. <br><br>Here’s the issue, his mom asked if I can add her to my list of close friends, I feel so so so bad not adding her bc she is literally my 2nd closest friends, but I feel so so so bad giving her access to my relationship with my fiancé. I just feel like that bridge is closed to parental figures and I don’t know how to explain that to her without her feeling like I’m excluding her or questioning our relationship.