MIL issues help please.

So my SO’s second mom (they are a lesbian couple and not his real moms just one of them has been in his life as a mother like figure since he was a teen) has previously created issues with me because she did not get her way with things. Well I thought she had moved passed it like she said she did. I haven’t had contact with her in months because of it. Well this week she comes over and she was fine the first day and we talked and she asked how things were going. Well the next day she comes over and asks how other things are like because are car tire got a nail in it (3rd time in the las 4 months). She ask how my SO got another nail in the tire and I said from work. She was like where is there construction around. I said either at the airport or at the college ( he is police officer and works two different places) both have construction constantly....

Then this is where I got pissed by her response.. she said, “ are you sure it is from work and not from something else?” With a face suggesting he was cheating. I told her no and she made a face as of yeah right.

My SO has no time for anyone else and I’m damn sure about it. He works two jobs back to back and comes home and goes right to bed for about 6 hours if that and gets ready for work. I told him what she said he looks at me and says where does she get the idea that I have time to cheat when I’m always at work and we haven’t seen either one of the moms for at least 6 months. Not to mention later on that day after she accused him of cheating on me she tells me that I should leave and go visit my family for a while. I’m sure she wants me to leave so she can put things in my SO’s head if i was to visit family. She knows that they are several states away and I feel like she’s trying to wedge in between us.And I’m pretty sure her wife has no idea what she has told me or the wife would not be happy with her. And it’s not like she could accuse me because i don’t drive and i don’t leave the house. We also have cameras in the house that my SO can check in whenever he wants. She know all of this. I’m just at a loss here I told my SO he should have a conversation with her and tell her to stay out of our relationship. Sorry for the long rant but help is needed.