Sister having baby by 1st cousin

So I just found out my sister might be having a baby by our first cousin. Months ago she told me it could be a 3rd cousin on another side of our family. She’d already slept w the third cousins brother prior to that. But now apparently it could be 3 others guys child too. I really feel bad for the unborn child. & ppl keep telling me I should get custody of the child (after she’s born) bc my sister physically abused her first child & still neglects her first. I’m a bit tied up & I would love to care for my niece & nephew but I have a 2 year & old & am 3 months away from giving birth to another child. Don’t worry, my child’s father is my husband of 3 years & NOT of relation to me!! Should I work on getting custody to make sure my niece & nephew are in a good environment?