Is this normal for a five month old GSD?

hey i just got a new GSD yesterday. She is very sweet, but the biggest problem im having is her being aggressive with my 2 yr old GSD. She growls and bares her teeth at him when he comes too close to our five yr old, or if he walks onto the porch when she doesnt want him to, or when he gets too close to her toys. She's not like that with people though..i have my daughter put her hand into her food when she's eating to make sure she doesnt react badly. but i thought a 5 month old wouldnt be this aggressive with others dogs this quick. any experience on this?