I have two boys, DS1 aged 6 and DS2 aged 3. Some close family friends have a 6 yr old who pretty much grew up (and now goes to school with) our DS1.
DS2 is 4 soon, starts school in September, idolises his big brother (i.e. can't even choose his breakfast cereal until DS1 has chosen), and continually 'punches above his weight' in terms of development, in order to keep with with DS1 and his posse (DS2 does have loads of his own friends at preschool, though). Most of DS1's friends totally accept DS2 as part of the gang.
Our friend's boy is an only child and won't be having any siblings. He has always taken DS2 under his wing and obviously acts out his fantasies about having a little brother on him. I've been generally OK with this - until now.
I'm getting increasingly p'ed off at our friend's boy's habit of monopolising DS2 and being horrible to DS1, in order that all the play goes his way and he gets DS2 all to himself. DS2, being young and immature, joins in and turns on DS1.
My usual way of dealing with it is to take my boys aside, hoping our friend's boy is in earshot (which he usually is), and saying I won't tolerate them being unkind to each other.
We were all at a do yesterday where my friend was having words with her boy, but as we were in public, I think she was happy for me to take my action to take the heat off her DS 
I feel that our friends partly encourage their boy to be kind/generous/whatever with our DS2 out of guilt for not being able to 'give' their boy a sibling. It's an easy route out of managing the anxiety/anger their boy has about being an only child. But this is starting to be at the expense of my DS's 'brotherhood'.
I can't sit back and let this happen when, a) I know that all three boys are capable of getting on great, and b) school beckons for DS2, and our friend's boy is already really excited. I know he will try and monopolise him in the playground, therefore taking DS2 away from his own friends, not just his brother!
Any advice? x