Is a person sinful if he obeys his relatives and family in matters not involving disobedience to Allah, in the same way one obeys one's parents?

Obedience to parents is obligatory as long as it does not involve disobedience to Allah Almighty. Obedience to relatives other than parents is also obligatory, provided it does not involve disobedience to Allah Almighty, because it mends hearts and strengthens ties of kinship. It is stated in Bariqat Mahmudiyyah by Al-Khadimi: "And in Al-Durar: Upholding ties of kinship is obligatory, even if it is just by offering salaam, a gift, or a greeting. It involves helping relatives, showing kindness to them, being gentle with them, sitting with them, and conversing with them… The paternal uncle, the brother, and the maternal uncle are to be treated like a father, and the maternal aunt and paternal aunt are to be treated like a mother in terms of reverence and obedience."