T.S.
L.,
I have two sons, 4 and 21 months. Quite honestly, I would step in BEFORE they do and correct your little guy when he does take a toy away from his cousins. Beat them to the punch, so to speak. At 2, he should be learning to share and we teach our 21 month old to share with his brother (and vice versa, lol).
About yelling at your son, no one has the right to "yell" at a child other than their own. Actually, we have taken the approach to not "yell" at our boys. In the situation you described, if another child were to take something from one of my sons as a baby, I would have gently said to the 2 y/o, "Honey, Ryan was playing with that. Could you please give it back and play with your other toy?" I'd give him/her a different toy in exchange for giving the one back.
If another adult did "yell" at one of my sons, I would very very calmly but firmly say, "Please don't talk to my son that way." I would then go over to my son and ask him to give back the toy that he took.
Good luck!
Edited to add: Please don't take my post to mean that we walk around with our heads in the clouds just talking calmly and nicely to our boys as they hit each other with sticks or throw rocks at each other. Lol. But, we save what I call our "Mom and Dad voices" for things that warrant a raised voice...like when my 4 y/o ran towards the street this weekend. Taking a toy away, IMVHO, doesn't warrant my Mom Voice. :)