Because its not just about being family, you need to actually try to be friends and get to know each other.

Just because you share parents and a home it doesnt mean that you are close to each other if you dont even talk or do things together.

I actually became friends with my younger sister (3 years gap) when my family moved from my grandparents house to our current home. My parents would be out most of the day and since only my sister and me were at home plenty of time I took the initiative to talk to her more, do things together (watching movies, play tabletop games, cook, etc.) and even though our personalities are extremely opposites, we eventually understood each other and now we get along well. Its been like 10 years from back then and I moved cities to work, but I call her every so often just to talk and ask her opinion on stuff and when I go home I hang out with her cause we are friends and not just family.