Is therapy an option? For everyone involved? I truly hope for your child's sake that everyone can put their differences aside and make this transition as easy as possible for your son. In the meantime can you talk to your sister privately about the important issues? As Gem pointed out, you must pick your battles. If it were me, I'd bite my tongue and say to my sister "I will never be able to thank you for all you've done for my son in his time of need. I'd like him to learn to express how he's feeling, will you please encourage him to verbalize how he's feeling."

Please remember that although your love for him hasn't changed at all, he's so young that he's had to adapt and your sister has taken on the maternal role. I'm sure he didn't establish that relationship with her overnight, and it'll take time, patience and a whole lot of love to re-establish it with you.

Ideally, this will all be supervised by a professional that will advise you all every step of the way. I'd also suggest you remember sometimes an apple is just an apple. Children that age rarely respond to disappointment well. Meaning that although you thought the plan was to watch a movie, he might have had a different one. Just because he's upset that he didn't get to do the activity he had in mind, doesn't mean that he's feeling abandoned.

I wish all of you wisdom and patience for this very long road.