E.B.
It sounds to me like you're dealing with grief over losing contact with a niece that you obviously love, more than wanting a baby. I don't have any idea why your sister or sister-in-law cut off contact with you, but that can cause grief and a myriad of painful emotions.
And I don't think that unresolved grief is a good reason to start fostering a baby. After all, fostering is often a temporary situation, and that would mean that you'd just have a baby in your home for a very short time, potentially. Fostering is best done, in my opinion, when one's resources are at their best. And it seems like you're dealing with grief that is affecting your life. Right now, you're weepy and sad (rightfully so).
Maybe you could volunteer at a hospital - they often need volunteers to hold infants. Philadelphia has a large children's hospital as well as many other hospitals. Or you could volunteer in other ways in the babies' wards.
But I suggest that before you commit to foster parenting, you face the loss of contact with your niece boldly. Either get counseling to deal with it if it's a permanent loss, or make attempts to reconcile with her mother, or figure out how to get stronger (emotionally) before making such a huge commitment like foster parenting. It's never a good thing to make a huge decision about anything (buying or selling a house, moving, quitting or taking a job, getting pregnant, taking on any huge life event) when one is dealing with grief or loss.