If you were a real Inuyasha fan you would know that he never groomed her. He explicitly said "follow me if you wish". He only saved her when she was a little girl because he was curious of tensaiga's powers. She was a companion just like Jaken was. However, he ended up caring about her because Rin showed him not all humans are despicable, just like how Rin thinks bandits are way scarier than demons because of sesh. They never had a father-daughter relationship. He never cared for her like a father would. She had to find food on her own and basically take care of herself on their travels. So yeah there isn't anything wrong with adult Rin being with him when she's older. It's the fucking feudal era and sesh doesn't age.