I am sorry your relationship ended.
Fortunately for you, this really is not a trans issue.
Unfortunately, the odds of it working with the twin are very slim. Chances are much greater that it will drive a wedge between the two of them and you will be the wedge.
And then, it could become a trans issue.
In the long run, both brothers would likely appreciate your decision not to date the twin. On the other hand, in the long run they probably would both blame you if you got in the way of their sibling bond... even if they each wanted to date you at one point in time.
I doubt many here will condone (support) your idea. But it is your life.
Good luck.