My DD (just turned 3) is OBSESSED with 'cuddling' her 5mo brother, and has been since he was born. She leans on him heavily when he's in his bouncy chair, she hugs him VERY tightly, she seriously invades his personal space during every waking hour they are together. When she talks to him, she puts her face about 2cms away from his and then shouts really loud. Now he has started playing with toys, she takes them off him all the time to 'show him how to use them' and forces his limbs into contortions so he can be in the right position to play with them (according to her!).
I constantly tell her to leave him alone, pull her off him, explain to her that he doesn't want to be shouted at, or yanked around etc etc. The phrase 'please be gentle' has been used so often that it fails to even register now! However, recently my friend and my SIL have both said that I should leave her to it unless she seems to be really hurting him. It is true that he very rarely makes a peep, even when she appears to be crushing him, but what would you do?