Fletcher-kun. The Fletcher from your brother's world still has him, remember? Sentience is making sure that he's being taken care of.
Maybe...just maybe the reason you were brought from different worlds was because you need each other? Sentience-san could have brought both you and your world's Russel...but he didn't. He wanted to make your worlds better, but he also wanted you two to be able to be happy. Fletcher can't be happy without Russel, right?
I'm sure...no, I know your brother back home is alright, as is Russel's Fletcher. But you two need to be together.
Family is what's in your heart. I know that. I think of Yuuri-nii as my older brother now, even though we're not blood related. As long as you love Russel, that's all that matters.