Need help with an abused chihuahua

So my aunt has an 8 year old female chi Lilly who I rly like but have trouble interacting with.

She was abused by her past owners, I won't go into too much detail but just to understand how bad it was, for the whole week before she got rescued she was locked in a completely dark room with a cat, they had just a single bowl of water for the whole week and a little bit of food. Even before that, she was being constantly yelled at and hit by her owners. (dw Abt the cat, she also has nice new owners now).

She was rescued to my aunt in late April. From then, it went way better than we thought. I rly care about her and I visit often, I love that dog as I love my own pets. But there is a problem. I had tried taming her down, she is very attached to my aunt, and sometimes lets me pet her, but whenever I slightly move my hand she sometimes strikes to bite or moves away. She also barks a lot whenever someone comes, gets up, goes etc.

I also have to mention she also lives with a 10 year old female shih tzu. They are very good friendsz play, cuddle and all that stuff but sometimes if I am petting the one and not the other at the same time they start fighting because that shih tzu can get pretty jealous at times.

But yeah, that's the current situation. She's 'ok' with me but I hope it will eventually get better, if anyone has any tips or advice I can do regarding taming her down or these two sometimes fighting please share.

Also, keep in mind that I can't control her lifestyle since she is tehnically not my own dog, but is very close.

Thanks in advance.