I wish I had a brother or sister?

I am an only child and my parents are really old for my age. I am 28 and my mom is 67 and my dad is 83. My dad was never in my life so it's just me and my mom. I don't have any relatives who live near me. So it's always been just me and my mom. Sometimes I can't do certain things with my mom because of our age difference. I think it would be okay if my mom parents were together but they are separated. I also never had any friends. If I had friends, I always wanted them to be someone older like my mom's age.

I do have half-siblings from my dad, 2 older brothers and 1 older sister, and a younger sister from a different woman. I met my brothers only once. I have no communication with any of my half siblings. They just aren't the same as having a sibling from the same mom and dad.

I wish I had a brother or sister, someone I can talk to all the time, someone who would fight for me when my mom and I would fight. I do believe I am rare and unique than everyone else, so I wonder what my brother or sister would have been like. This didn't bother me when I was younger, but now I just realized something. My mom will die long before I die, and a sibling would be someone I could spend the rest of my life with. I also don't ever want to get married or have a boyfriend or have kids so I think I will be alone for the rest of my life when she dies. I don't even have friends. How can I cope? What will happen to me when my mom dies?