Is distancing oneself from one's parents for a period, due to their favoritism towards a sister and speaking ill of their daughter, considered an act of disobedience, even while supplicating for them in every prayer?

Filial piety is among the most obligatory duties, and disobedience to parents is among the greatest sins. If parents prefer one child over others, they are mistaken, but their error does not permit the child to abandon them or mistreat them. Allah commanded good companionship even for polytheist parents. Abandoning parents is a forbidden act of disobedience. Therefore, you must take the initiative to show them piety and kindness. You can advise them gently and supplicate for them, and responding to ill-treatment with kindness engenders affection.