So i haven't went in detail of what mf Heidi and Howard relationship is like here, so I'll explain their dynamic.
Compared to the series and my rewrite of the siblings bickering over everything, in the mf au Heidi acts like a nice but overbearing towards Howard. She doesn't mean to do anything in malicious intent she just wants to spend as much time with her brother as possible. They used to be really close when they were little but as time went on they grew apart and Heidi wants that bond of him needing her back.
When he started to transition she was the most supportive sister ever. Finding him clothes, correcting bigots, and even loudly Proclaiming every time he achieves something "THAT'S MY BROTHER HOWIE!"
She means well but Howard finds it annoying that he can never have alone time to himself, he loves his sister but she doesn't actually know him, all she sees is the adorable sibling she knew when she was little basically infantizes him.