The 10 year old is a "Tween."
Tweens are from 9-12 years old. He is essentially a Pre-Teen.
Google Search "Tween Boy Development" and read it well. Many articles will come up.
Now, you hint at an issue with Grandma... and her having "joint" custody of Collin etc., and how Collin is always at Grandmas.
Well, is she unfit as a Grandma? Is she toxic and weird? Is she psychologically unfit? Is she bad mouthing you or Nicholas to Collin? Is she manipulating him?
Or does she basically, FAVOR Collin over Nicholas and you and everyone else????
Then, there are sibling problems between the boys. Well, you need to also realize, that Collin is now 10 years old. He is a Pre-teen. Big difference from a 7 year old. They have different interests at this age.
But yet a younger sibling still wants to tag-along with the older one. But the older one gets irked by it. Typical. There are Age differences now... which are more obvious because Collin is now 10 years old, and not a little boy.
My kids are 4 years apart, with the oldest being my daughter. She and her little brother are very close. BUT... she is also 9 and changing and at times... she just wants some privacy and her own time, without him. Yes, my son gets sad or does not understand, he is only 5 years old. But, we as parents, realize that my daughter is hitting that age phase... of being a Tween... and it will... affect her younger brother. Because, she will become more independent in terms of just age phases, (and she will do more things without him) etc.
But the thing is: my kids are really close. And, they know its not nice to abandon the other for selfish reasons.
Perhaps, talk with Collin. Don't scold or blame or anything. But you are his Mom. Grandma is not. You are the Mom.
You need to talk with Collin and make sure he knows, that you/Nicholas/and himself, are a TEAM. And you all care about each other. Not hurt each other's feelings.
I don't think Collin realizes, that him being off on his own all the time... is making Nicholas feel.... lonely.
That is why they fight.
When my daughter's older cousin was in town... she IGNORED her little brother. And my son told us, he was LONELY. AND he cried. We had to speak to our daughter... and she didn't realize. She was just thinking about her cousin.
Or is Grandma putting ideas in Collin's head? Since he is there so much? And he is getting... self centered?
You need to... make sure that Collin listens to YOU.... not Grandma all the time.
YOU are the Mom. His Mom.
And Nicholas is his little brother. He needs to watch out for him. And care.
I think the whole Grandma thing... is too much.