why did that old guy had a 11 year old sister?

I looked it up, and their respective ages at the movie's release were 54, 38, and 14. So yeah, quite a bit closer in age to the mom. Means his mom had him at 16 and his sister at 40.

So. Mick Jagger has a son who is younger than his great-grandchild.

https://www.cnn.com/2016/12/09/entertainment/mick-jagger-family-tree-trnd/index.html

Not unheard of. Especially back then. My Great-Grandmother has thirteen children over a period of 21 years.

I once knew someone who had a sibling in their early 30s while they were in their teens. It's not unheard of at all, even if it is a bit odd nowadays.

My husband's mom had her first child in 1949 when she was 19 and her last child in 1973 when she was 44. It happens....