Kitten at play

I adopted two kittens from the same litter. Their estimated age was 3 months old. They had been found at 6 weeks old and had been cared for (shots, loving attention, etc.) for the other 6 weeks. Both are females. Mercy is the runt and is still battling UPI (Upper Resperatory Infection), but Grace seems to be a first born. She walks away and gives Mercy her food when Mercy has finished her own, and seems sad and troubled and runs away and hides. Most of the sad and troubled has gone away and we have done many things to socialize her and make her affectionate, including cuddling sessions- she's done great!

But, the reason I am writing is that there is a toy that she seems to enjoy-she can play with it for hours, literally. However, when she does she seems to get very territorial and I won't even go near her. She holds the tassie in her mouth and looks at me as if I were going to take it from her. She growls and hisses and then goes on playing. I love to watch her play with this toy, but I'm afraid of her becoming too aggressive in general. She does not show any of these behaviors with any other toys. She plays well with Mercy any other time. Can you help? This one has me baffled!

Don't panic just yet! She is just using her hunting skills. This will not cause agression in any way. Have you tried to get near her when she has it? They usually wont act on the growing. It is something even cats who were born in houses will do. I usually get a kick out if it when it happens here. It's like watching them in their own natural setting.