Is it normal to cry when she leaves?

ok so here is the story. me and my sister never really get along and she 17 and im only 13. like at points it can be fun but she likes to argue and i hate it. but she is always telling my mom that when she turns 18 shes leaving that second and not looking back. when ever i hear about it or think about it it makes me want to cry. and i do cry. shes really cold hearted andoesn'tnt really care what others think and the other day i was sitting on my bed crying and she walked in and asked me what was wrong. i had to make u a libecausese don'tnt know how she would hav tooken en it. i realy don't dont know what to do. help me out here please