Does a sister's failure to maintain ties with her brother, who stole from her, constitute a severing of kinship ties, and what should she do to avoid that?

Severing ties of kinship is a grave sin. Boycotting and shunning a Muslim may be for a worldly matter, and it is not permissible to boycott a Muslim for a worldly matter for more than three days, according to the Prophet, peace and blessings be upon him: "It is not permissible for a Muslim to abandon his brother for more than three days." The boycott is lifted by greeting him with peace. If he does not return the greeting, one who is better than him, like the angels, will return it for him. As for boycotting for a religious matter, it is permissible if the Muslim insists on a sin or innovation, and it is hoped that his boycott will lead him to repentance.