Thanksgiving…

So my partner and I have been together for going on four years and my MIL didn’t have any problems with me in the beginning of our relationship and then everything changed one day when my partner was over at her house and had asked me to come and pick him up. Since that day she called me names like a b**ch a wh**e and many more. I tried talking about it with her and she just said she thought I was too possessive and clingy which whatever to each their own opinion however shortly after saying and I quote “never let my partner have more than 2 minutes alone” we announced my pregnancy to which she started saying things to other people in her family like I cheated and the baby wasn’t his that I was drinking and doing drugs while pregnant and making up all these lies about me. However to bring you up to current time the day after I had him she came to the hospital acting as if we were best friends. Come to thanksgiving well my partner and I switch Christmas with our families or are supposed to however the first two years we went to his families Christmas and last year I was sick however this year I told my partner either we will all be going to my families Christmas or our son and I will be going to my family Christmas to which he then decided that he all of the sudden wanted to go to his families thanksgiving even though we had plans already for thanksgiving anyhow I cancelled our prior plans to please him and went to his families Christmas. While there my MIL took it upon herself to TAKE my son out of his aunts hands KISS HIS FACE and REFUSED to give him back to me. The entire day of thanksgiving she was passing my son around to all of his family members after my asking her to stop due to it being RSV season. I voiced my opinion asking for my son back however my partner quickly shot me down saying “we get to see him all the time let him meet my family” which I understood where he was coming from and normally I would have no problem with people who aren’t sick or have washed their hands to hold him however KISSING HIS FACE AFTER HIS MOTHER SAID NO!!!! Not only is that just weird in general but very disrespectful and very dangerous to my son. After having done that she keeps insisting to be his babysitter so my partner and I can go out he would love that and I would like to spend some time alone with him however I will NOT leave my son with her EVER !! How can I get this through to her without sounding disrespectful as to avoid an argument with my partner?