Is it permissible to refrain from greeting one's stepmother with "peace" (Salam) if she does not return the greeting, and to avoid sitting with her, knowing that she harms the questioner only with words, without that being considered a severing of kinship ties?

You must be patient with your stepmother's harm and forgive her, and you will be rewarded for that. Your right to greet her with Salam is not forfeited if she does not return the greeting; rather, you should always initiate the greeting even if she does not respond, because Salam severs estrangement. You should also explain to her that returning the greeting is obligatory. You are permitted to avoid sitting with her to avoid her harm; this is a beautiful avoidance that carries no blame.