If Rin is a kid then does at some point she will be a teen at the end of the onyesha and isn't Sota Sesshy's brother in Law? And And why can't Sota go to the other realm?

Rin is only 10 years old at the end of the manga/anime, after three years pass then it ends.

The brother in law thing *no comment*

And Sota can't pass through the well because he never had any jewel shards in his possession. It's the jewel that allows passage to the feudal age, that and Inuyasha's Enchanted beaded mkufu because it is connected to Kagome. So long as she had jewl shards in her possession he can follow after her to modern times without the need for shards.