Hi Jasmine,
Welcome to our community! Boy, you like to start off with the hard questions!
The truth is, no one knows for sure yet. There simply isn't enough research into that situation yet. We don't even have access to accurate testing yet for those of us that are already here.
It's believed that with the right antibiotics (abx), the transmission from mother to child can be prevented, although that's not proven either at this point.
Not all children born to mothers with active infections will wind up with infections themselves - my children (now 30 and 31) have never had these infections, yet I carried them while I was untreated and infected. It's just that the risk is there, and of course we all want healthy children to be born. And there are other mothers that have unknowingly been infected with Lyme while they carried and gave birth to children.
Your brother in law's wife, once her infection is in remission, could possibly carry a child to full term on her own, while taking abx (usually amoxicillin) to reduce the risk of passing it on to the child.