Does anyone have advice?

My daughter is 3, we're expecting a little baby boy next month. She's kind of the jealous type which makes no sense because she gets a lot of attention (we have a big family) & she gets tons of attention from my fiancé and I. BUT thing is my fiancé has a niece and anytime she's over they tend to not always get along.. Well you see, the niece is 4 and doesn't always share either so me teaching my daughter to share doesn't always work because the niece tends to rip things out of her hands and things sometimes when people aren't looking which can be irritating.. How can I get my daughter to understand about her brother? She understands her brothers in my belly, but I've always heard of siblings feeling less important or not coping well with a new baby. Obviously she would still get a lot of attention that would never change, I just worry. Advice?