I would disagree with the majority's opinion here. I understand you might initially feel sidestepped, but it's not like your dad married you to Tristan or invited him to live with you against your will. I mean - if you don't want him to move in, you can always say so, or you can ask him to move out at any time. From what you write about Tristan, I believe he wouldn't move in unless he asks your opinion as well as your dad's and would move in only if you're ok with it. In so far, he only got your dad's 'no objections' opinion, but you can put any objections you want.
You're adult and you have a job, you can move out without anyone's permission. But,... do you really need financial difficulties before your transition is over? And from my understanding, you really like to live with your dad and brother, so why moving out if you can be together with your family and your BF?
I mean, it would be smart to poke Tristan a little and make sure he realize that you shouldn't be sidestepped in the future, but if the current outcome suits you, why creating a problem when none exists?
At least, wait another year or two, when your transition is over, Tristan gets some work experience, you build up some savings, then you'll be ready to have a place of your own.
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