Is falling into major shirk considered to be accepting the advice of one's parents and siblings out of love for them and obedience to one's parents?

Obeying and loving one's parents is not polytheism (shirk); rather, it is something commanded by the Sharia. Allah has coupled gratitude to Him with gratitude to one's parents, as He Almighty says: "Give thanks to Me and to your parents; to Me is the final destination." Their obedience is in what is good, not in disobedience. And the rights of righteousness and kindness towards them remain, even if they command polytheism, as Allah Almighty says: "But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with appropriate kindness." If you comply with them in a sin, this is not polytheism, but rather a sin that requires repentance with its conditions.