Okey dokey smokey...lol. You asked for advice and I will give you my two cents worth...
We have been in this exact situation as you are probably aware. Keyana's big brother is 3. We have kept him before though not for an entire weekend. He can be a PITA. He has acted out towards Keyana when in our care and we refuse to allow it at all.
We dont hurt him or anything but he gets treated the same way we would any of our kids. Just because he isnt a blood relative doesnt mean squat to us. We actually like the little guy. He is just a bit on the undisciplined side. We dont let him get away with his antics. So who cares if he gets upset? We punish him. He screams and yells but he gets over it and is Papa's buddy in no time.
My advice would be to try and find some activity that can be extra special between your husband and "grandpa's boy" alone. Make sure it doesnt involve the girls at all. It can be as simple as feeding an animal outside, cleaning out a car, checking something outside, but make it a "boy" thing. Make it something that they do everytime he sees Grandpa.
For instance, whenever Cameron sees us, he has to go out and feed the chickens with my husband. Doesnt matter if they have just been fed, they go out anyway. Even if its dark, they go out with a flashlight and look at the chickens. He always talks about the chickens and his Papa.
