Daughters Bio Father Is Threatening To Claim Her On Taxes

So, me and my wife and I have one daughter (f 1yr), and she and her ex had a daughter while they were married (f 6yr). Me and my wife have physical custody with visitation "as agreed upon" as well as his child support "as agreed upon" *per their divorce document). So, in reality, it means he asks for her when he feels like it, and we work with him to make it happen, and he pays 200$ a month. I'm very aware this is a shit show legally, it was a messy divorce (he's a man child who throws fits and likes to threaten to kill people when they don't do what he wants) so she was desperate to get it over with while keeping my step daughter as much as she could, and these terms were the only ones he would agree to so she did it. I have begged her to go to court to make things more concrete as this feels like it's riddled with chances for him to exploit if he actually wanted his daughter more, but thankfully he doesn't really care about her. But, this tax season has him upset that he had to pay. Me and my wife claimed her last year and plan to do it again this year. Again, we have 95% of the time as he just asked for her once every few weeks usually. He only pays 200$ in child support, so we didn't think us claiming her was even going to be a question. He is now demanding to claim her this year and says he is going to do it no matter what. What are the legal ramifications if we claim her and he claims her? I am hoping that with us having custody, it will just get waived our way, but I wanted to check with yall to see if there will be a big Hassel.