Is a girl's refusal to attend the gatherings of her family's madhhab, after changing her madhhab to that of Ahl al-Sunnah, considered disobedience to her parents, and is attending these gatherings forbidden?

You must avoid gatherings of iniquity that involve polytheism (shirk) with Allah, such as invoking others besides Him, as Allah Almighty says: ﴿And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they engage in another conversation. Indeed, in that case, you would be like them.﴾ So, being pleased with disobedience is disobedience. And there is no obedience to parents in disobedience, as the Prophet (peace and blessings be upon him) said: "There is no obedience in disobedience; obedience is only in what is good and right." It is possible to avoid outright refusal and use dissimulation to avoid harm.