I've not been in your situation, but I'm the (adoptive) parent of two children, DC1 placed with us as a toddler, later DC2 placed as a baby, same birth mum. It was documented that BM drank during both pregnancies but the children were largely meeting developmental milestones and FASD was never mentioned when they were matched to us.
Youngest is now at primary school and after a very difficult few years has a diagnosis of full FAS and eldest is being assessed for possible FASD too. Their needs are so great I've not been able to return to work due to the many appointments & meetings I need to attend, night waking, difficult, impulsive, risky behaviour etc. DC2 can't be left unsupervised for more than a minute and their different difficulties become more apparent as they get older. We love the bones of them both but parenting them has really tested our mental and physical health and our marriage.
It's lovely that your niece has become such a part of your family but she and the new baby could go on to develop difficulties as they get older, and six children, however they arrive in your family, is a lot, particularly if some need support with disabilities.
As others have said, there's no "right" decision here, it's about what works for you and your existing family. The new baby will be found a family if you make the decision not to take them on. Sadly it sounds as though there could well be subsequent children too who will not be able to remain with their birth family.
Best wishes whatever you decide to do 