Hmm. I think that it's perfectly reasonable for your daughter to be upset because her american girl dolls are very expensive and as you can imagine, a two year old would probably ruin the doll. I say, next time her two year old cousin comes over, tell her to put her American girl doll away. Have a conversation with her to let her know that while it is okay to not share her American girl doll because she is afraid that the two year old would ruin it, she should share her other toys when the two year old is older. You should make a box of toys to share for the next time the two year old comes over and have a conversation with your daughter about the toys she's not willing to share. Hearing the reason is very important. For example, saying that i don't want to share my dolls because they are MINE is very different from I don't want to share my dolls because they are precious to me and I'm worried that the two year old would not know better and damage it. Make sure to reward your daughter when she shares her toys with her little cousin but hear her out if she doesn't and as long as the reason is reasonable, tell her that you understand her reasons but that sharing is a good thing.