BPD sister is off her meds & wants to have a baby

My sister (35F) is diagnosed bipolar with paranoia type anxiety, along with major depressive disorder and ADHD. It’s been a lifelong struggle for her and our family (I’m currently in EMDR therapy, working on my healing).

For context - she was living on her own for the past several years but only w/ extreme support from my parents. My mom would drive 30 minutes to her house every day to ensure she was up/ready for work, took her meds, cooked all her food, did all her laundry, and even cared for her pet.

~1.5 years ago, my sister started dating someone, they moved in together after a few weeks, and then they got engaged a few months ago. Her fiancé had taken over all domestic tasks that my parents used to manage for her.

My sister and her fiancé have recently decided they desperately want a child. However, I am concerned for two main reasons:

1. My sister’s fiancé does not know about her diagnosis

2. My sister has also recently gone off her medication (Since going off her medication, her mood has gone erratic, so I imagine her fiancé likely has some idea of her current mental wellbeing).

The thought of my unstable sister having a child absolutely terrifies me. I think of the abuse and mental anguish I suffered at her hands as a child, and my anxiety reaches unparalleled levels when I think of her carrying for someone as vulnerable as a child. Though I try to remind myself she was also a child when this occurred, it is not of much comfort to me because from my perspective, she has not changed/grown very much, if at all.

Ok this has gotten really long so I apologize and thank you if you’re still reading - I guess I am shouting into the void because it’s hard to talk about this with my husband or friends, because they are not as intimately familiar with the complexities of bipolar disorder and/or relationships with those diagnosed.

Any ideas, advice, or thoughts welcome - or perhaps if you have gone through something similar I would love to hear how you felt and what you did? Thank Thank you in advance 🫶