It's called a bicornuate uterus. I don't know much about them but I'm sure a search on Google will turn up quite a bit.
I know that women with such uteruses have a higher miscarriage rate and a higher rate of complications, be it breech presentation or premature delivery. But that doesn't mean everyone - some still go to term and deliver fine, healthy babies. I think it depends on where the baby implants, and if the side of the uterus the baby implants in is able to accommodate them through the 9 + months.
ETA: Sending prayers up for your SIL.
I know that women with such uteruses have a higher miscarriage rate and a higher rate of complications, be it breech presentation or premature delivery. But that doesn't mean everyone - some still go to term and deliver fine, healthy babies. I think it depends on where the baby implants, and if the side of the uterus the baby implants in is able to accommodate them through the 9 + months.
ETA: Sending prayers up for your SIL.