Laura
I know what you mean to him because I felt it. I saw into his memories. Felt his fears and his worries. He wants so much for himself but he can't have a life because he worries so much for you and Cora. As long as you two are here he has his obligations and I admire him for that, for sacrificing what he has to because his concern is always for his beloved sisters. I was the same way. I would do anything for my twin brother if he were still alive.
He's still Derek. He may be acting odd but he still wants me. Still seeks me out so I will use that to figure out what happened to him. If I sense something seriously wrong with him I will let you know immediately. I can reach out to you. You'll just have to let me in.