Abbie's Mom

I have a wonderful 8 mo. Old shih tzu. She is a great dog. She’s very gentle and listens very well. She was actually very easy to house train. My problem is this: she is showing signs of food aggressiveness towards our other dogs. She has actually snapped at our lab when getting treats. Also, some “attention” problems. If I am holding my sister’s Chihuahua/Yorkie mix, she seems to send her vibes, like, “get off my mom” although she hasn’t snapped at her, she is on her toes and watching her the whole time. How to I break her of this before it gets to far? Any help would be appreciated.