Am I being irrational?

I think I mentioned before about my future SIL? Before our babies were here I think. She has bipolar II with rapid cycling (manic depression that goes in quick cycles so she will be depressed and manic many times over the year). My brother and her are getting married in a years time and will be living in my parents house for the first year. My issue is that my mum will be looking after Lucas one day a week when I go back to work in August. My FSIL will be there more of the time as she also has family issues (her sister has severe OCD, her mum depression and a severe attitude problem and her dad is an alcoholic!!!). I have always voiced my concerns about her ability to look after Lucas. She loves him to bits but I can never quite be sure if you can trust someone with a baby who has a severe mental health disorder. She has on occassion trashed her room and tried to kill herself or throw herself in front of traffic etc. She has never had an episode in front of me and acts normal most of the time. She hasn't had a very severe episode recently but I know she is currently in her manic state (especially getting obsessed over this wedding).

I can't help worrying about it and have told my mum and brother but they both think I'm overreacting and I have to stop worrying but I don't think it's totally unjustified. My mums reasoning is that she has been oked by the council to work in after school clubs after consultation with her doctor but thats totally different as she would never be alone with a child. I don't want to make thisa family dispute because I know that it would probably end up with me putting Lucas in a nursery and my mum not talking to me! Although, my brother goes quiet when I talk about it so I think he probably agrees with me but goesn't want to say anything.

I really want to sort this out before august but I don't know the way forward. I might email my brother and talk to him. I haven't approached her as I don't want her to feel bad because I know she would never intentionally do anything to hurt Lucas. She does sometimes check my posts (which freaks me out a bit as she has no kids and isnt planning any) so I'm hoping she won't see this as she is on holiday at the moment. If she does I hope she tells me so we can talk about it. I might have the courage to tell her one day but right now I don't want to hurt her feelings. It must be awful having something you know you'll have for the rest of your life and it willeffect everything- don't even talk about her and my brother having kids! She will have to go thru medication changes and close monitoring and everything!

Anyhoo, that was along post but I just needed to get this out. I'm feeling so alone with all this!