Hi L.. Sorry to hear about your situation. I have an ex of 13 years. We have a 14 year old daughter together. In my situation I have sole custody & she goes to visit him. Our papers say she is to go 1st, 3rd & 5th weekends, every Wednesday night, on a specific day at Christmas, a certain amount of time over the summer, etc., however, in the last few years we haven't followed that schedule. She just goes when she wants to go, which is way less often that what the papers say she is "supposed" to do.
With your situation, the fact that your child is 14 has a lot of weight. The judge will listen to what you and your ex have to say and will likely talk to the child. They judge will listen to reasons why your ex feels like he is a fit parent & why child would benefit more from being with him, the judge will listen to why you feel custody should not change, and then they judge may talk to the child for more info and feedback since it is the child's life.
The fact that there is another child living with the ex who has problems doesn't matter in the custody of your child. Nothing matters unless you can prove beyond a reasonable doublt that your child will be in NEAR DEATH DANGER if your ex gets custody. Don't ask me know I know. I have been dealing with a crazy, violent ex for 13 years now. The most I have ever been able to get is a TRO (temporary restraining order) to keep my daughter safe, but even during times outside of the TRO when he was violent, my daugher still had to go see him becuase he hadn't already nearly killed her.
On the flip side,depending on how long you have had custody of your child, the jugde is not likely to change custody NOW if you have always had custody. The judge is also going to look at stability for the child.
I know a lot of this isn't what you want to hear, but unfortunately that's pretty much how it works.
Good luck with your upcoming court hearings! I wish you the best!