Sister in law cut off how do I explain to our child

My sister in law wants nothing to do with her brother or me or our child..she is very jealous that we are having a little girl and she's jealous that her mum never gave her things that she's buying for our daughter..my sister in law is 18 and makes out she's got a really hard life ! She hasn't she's had a lovely upbringing in a lovely home!! She hangs around with rough people and has a bit of a reputation! I'm wondering what do we say to our little girl when she's growing up? How do we tell her that her auntie doesn't want anything to do with her because she's jealous of what she has!? Just to add I'm still pregnant and she's not even born yet! I have no idea how she's jealous of an unborn child..she's annoyed that her mum is happy to have a grandchild! What would you say to your daughter?