Discovered a half sibling on 23andMe, should I tell them who the dad is?

Long story short, I discovered a half niece who reached out to me…meaning I have a half brother out there. She knows either my dad or uncle is her grandpa but I told her I would find out. My dad had no idea of this child, this happened nearly 50 years ago. It was before my mom. Her grandma lied about the father this whole time. The supposed father has blonde hair blue eyes while they have dark features. The grandma said it was the partial native side when really it’s because they have Asian ancestry.

The girl responded back, hasn’t told her dad or grandma her discovery but said she wants to meet my dad. After my talk with my dad yesterday, it sounded like my dad wanted to leave it alone because my mom would be devastated even if it happened before meeting her My sister thinks it will harm her health since she isn’t in the best condition.

The niece said she would be happy to know where she came from and who the grandpa is even if they don’t meet. 2 of my siblings and I feel we should tell her who it is to avoid her from contacting other relatives and that it’s best via us. There is a chance she may go to her grandma and confront her, and her grandma tells her.

My 2 older sisters feel differently. That we should respect my dad’s wishes and for the sake of their marriage and ignore her. That dad said he would deal with it if she digged for more info herself if it got to that point.

I don’t know what to do.