I watched the series for nostalgias sake because Inuyasha was the first anime I really loved as a child. I think it makes sense Rin and Sesshomaru ended up together once Rin became of age. The story has sadly unfolded to be horribly boring... This is not the future I hoped to see for these beloved characters.
There are huge missed opportunities with this series. The romance has always been a center point. Something that is truly tragic with the demon/human relationships is Kagome and Rin will grow old while their demon husbands will remain relatively untouched by time. I thought Rin was trapped in the Tree of Ages because Sesshomaru might have been trying to find a way to extend her life. This is touched upon in the original series when Rin asks Sesshomaru if he will always remember her after her death.
I think a great way to deal with this situation would've been Rin deciding to go on a journey of her own because she wants to be with Sesshomaru forever as she stated many times in the original series. This would've allowed her to escape her damsel in distress archetype while creating a exciting new plot. This could have happened even after the birth of her children with Sesshomaru watching after the children from afar and not knowing where Rin had disappeared to. Or maybe Rin agreed to be taken away to protect her children.
A great way to allow Rin to achieve her goal of staying with Sesshomaru for eternity would've been her finding a way to become a demon herself. I think this couldve been a really badass twist and allowed a new respect for adult Rin.
As for Sesshomaru sealing Inuyasha and Kagome away, leaving their child orphaned... Absolutely awful writing.