Will the wife be sinful for not speaking to her irreligious stepmother-in-law, who disregards everyone's feelings, after many attempts to get close to her?

It is not permissible to abandon one's stepmother for more than three days, except for a legitimate Shar'i reason, such as if abandoning her serves as a deterrent from an evil act, or if maintaining ties with her would result in religious or worldly harm.

Part of showing filial piety to one's father is maintaining ties with his wife and honoring her. Responding to mistreatment with kindness brings affection and removes grudges.