Should I Interfere?

I have a parenting dilemma and am not entirely sure what to do!

Girl is 4 and a half, Boy is nearly 3 and although they love each other dearly and love to play together, their play is still punctuated by yelling, screaming and fighting!

Quite often they will be playing together in Girl's bedroom and all of a sudden there will be squeals or yells from Boy as his big sister either tries to make him do something, sits on him, or just takes something off him!  My natural instinct is to interefere but if I leave them he doesn't come running to me telling tales, unless she's really hurt him, and soon the squeals stop and he's laughing at her again.  He's a stocky lad so I doubt she could hurt him.

My dilemma is this: should I interfere as sometimes what she is doing is tantamount to bullying, or, as he doesn't feel the need to get me to interfere and will be playing again quite happily a few moments later, should I leave them to get on with it and accept that this is something they have to learn to work out and will be better people for it?

Any comments would be gratefully received!

PS:  To date there has been no actual damage to either child!