My dog has become aggressive and has went against me. Need advice

 I have a dog named Brownie and he is a mixed breed consisting of a chihuahua and something else. I had my dog since he was a puppy and he grew up with me during my childhood. My dog is very friendly with people but we taught him a bad habit of growling and barking at a person who seems to be threatening someone else. My family and I don’t abuse my dog because he listens to whenever my family and I call him or tell him some thing and also we just don’t like hitting the dog at all. My dog is currently 7 years old and I am 20. The story begins when my family and I decided to go to Mexico for the summer with our pets for the first time. As we arrived at our home, my step dad, step brother and step sister (they have two dogs) were waiting for us at our home. My dog met them and he got comfortable with all of them immediately. However, as 3 weeks went by, my dog got heavily attached to my stepsister and tried humping her leg multiple times. He now just follows her when he is in the same room as her, cries when she is gone or in another room and breathes heavily and has his tongue out when he is with her sometimes. The problem began when he began growling and barking at anyone in my family when they tried taking him away from her or even go near her. He began to guard her, follower her non stop and defend her. This was the first time I’ve ever saw this type of behavior from my dog and I decided to take action. At first I did simple calls like “come to me” or “let’s go outside for a walk”, he look at me but ignore me. When I tried removing him from her, he growled and barked at me heavily, showing his teeth and everything. I was able calm him and was able to successfully remove him from her. However, at one attempt where I grabbed him, he bit my hand hard and didn’t let go and at the heat of the moment I opened his jaw to release his teeth from my skin and slapped him. When I slapped him, he bit through my skin and he ran under the bed to hide. I went to clean up the wound and the little bit of blood that came out and went to relax for a bit. I realized what I did was wrong and decided to not do that again because I don’t hit my dog and i know I can do something different this time to avoid this. Some actions I took was to tell my stepsister to command him to come to me and to stop paying him attention but he still keeps doing the same thing. As of right now, he no longer listens to any of my commands when he is with her and I am getting frustrated now. We are leaving Mexico in 2 weeks and I am asking if anyone has any advice on what I should do. I don’t like his behavior and don’t know how to stop or control it because this has never happened to me. Any advice would be helpful because I feel like I’m losing my best friend. 

submitted by /u/4KFelix
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from Pets https://www.reddit.com/r/Pets/comments/oz95ay/my_dog_has_become_aggressive_and_has_went_against/