The preponderance of terms like "girldad" and "boymom" among cis people offers a clue into parental transphobia: they literally see their children's gender a part of their identity. "Well actually you're the one forcing me to take on an identity I don't want" is a DARVO, but it's a darvo supported by normative ideas of how parenthood should function.
but yeah i think this kind of thing is why gender reveal parties feel kinda gross to me. well, that and the explosions and wildfires
anyway uhh clearly the solution is for siblings to transition in pairs. swap genders. /s