Am I sinful if I stop visiting my husband's family and asking about them, given their ill-treatment of me and their failure to inquire about me during difficult times?

The marital relationship is based on affection and cooperation, and each spouse must help the other maintain ties of kinship, because severing ties of kinship is forbidden and prevents entry into Paradise. Allah has commanded us to maintain ties of kinship even with those who cut us off or treat us badly.

Therefore, do not hesitate to maintain ties with your husband's parents and siblings, seeking the pleasure of Allah and out of kindness to your husband, even if they treat you badly. This is not an obligation upon you, and rejoice in the blessings of this connection in this world and the hereafter if it is purely for the sake of Allah. Indeed, your maintaining ties with them is the best remedy for their ill-treatment, in accordance with the Almighty's saying: ﴿Nor can goodness and evil be equal. Repel (evil) with that which is better: then verily he, between whom and you was enmity, (will become) as though he was a close friend.﴾