Worried about MIL

Jennyfer • currently pregnant with baby #2. Mother to my 3 year old.
My MIL has schizophrenia, anxiety, and bipolar disorder. She's normal when she takes her meds but lately she's been off them and kind of freaking me out. Idk what to do. My SO doesn't seem to worried because he's used to her being that way. She didn't get diagnosed till he was 18. She always talks about taking care of our baby when he/she is born. I'm just really scared to leave my baby with her because she sometimes doesn't take her meds but I don't want to tell my SO because I feel like I'll offend him. Idk what to do.