My mother is spreading lies about my husband

We are a multi gen family- my parents live with us. We decided to do this when I got pregnant, my husband and I were living in an overpriced apartment with thin walls and were worried about neighbors noise complaints after baby came, etc.

My parents had just remodeled their home and it was just the two of them in a 4 bedroom house. We pay all our own bills, rent, etc. We get free child care from them when we need it, sounded great the thought of it.

My mom is getting older, 66, which I feel isn’t ‘that’ old, but she has a drinking problem and is now making up stories about my husband. Problem is she believes them and tells my sister and my sister then probably tells her boyfriend.

So our part of the house is 3 bedrooms (office, baby room, master) but only one bathroom. The one bathroom is in our master suite. When my mom isn’t home, I ask my husband to go #2 in one of the other bathrooms, which is on ‘their’ side of the house. Reason is obviously because it would stink up our master bedroom and that’s usually where I am hanging out.

The other day, my husband thought my mom left to go run errands so he went to go poop in one of the bathrooms on their side of the house. Apparently, my mom was home, and walked in on my husband taking a dump. He immediately got embarrassed and apologized and finished his biz.

Now my mom is going around telling ppl he exposed himself to her. I’ve asked him 10 times if that’s true he swears up and down that is completely false and was mortified and didn’t even know she was home. But my mom won’t let it go. I ask her exactly what happened and her story changes a little every time.

My husbands version is the exact same every time I ask him. I think she’s getting Dementia or something. But the damage has been done, my sister who doesn’t live with us has been told my moms version and they are closer than my sister is to me. I don’t want to be one of those women blindly defending her husband but his story seems more accurate than hers. I also now feel like I need to take my mom to see a geriatric Dr. but am mad at her for spreading rumors. Advice appreciated.

Edit- My mom is a night drinker and absolutely does not watch our daughter when she is drinking. My dad doesn’t drink and is there to supervise in the day time. Most of the time either myself or husband is home but there are times e.g. dentist apt. when they will watch her. This isn’t a childcare issue it’s more my mom having a power trip toward my husband. We cannot wait to have the means to move but his job is in the city and rents have gone up so much since we moved in here plus baby expenses. I am looking into affordable housing but in my area the waitlist is 2 years, and also nutritional subsidies. We make $72k per year which is basically poverty level in Los Angeles.