What is wrong with me?? Why am I so jealous of my girlfriend's family?

My girlfriend's sister and her family (her husband and son) are coming into town for a week. The son is about 2 and she is pregnant with another. I have always been extremely jealous of her relationship with her sister and her family and I don't know why. I have been dreading this week that they're going to be in town because it means I won't be able to talk to my gf as much and all of her attention will be on them. I get angry and annoyed whenever she says she's excited to see them. I feel so stupid for being jealous of a literal baby and I have no idea what's wrong with me or how to fix it. I don't want to feel this way but I always have. I want to be able to have no problem with her sister coming and knowing she won't have as much time for me. We are long distance at the moment (college), so it's harder when I don't get to text/call her as much. What should I do? Why am I feeling like this? How do I fix it? Please give serious answers -- I know this is unfair and toxic and I want to fix it because the way this stuff makes me feel is painful and I don't want to feel it anymore.