Why should you feel guilty? you can't pick you family, she needs to grow up,we can all make excuses for how we behave, just get along with your dad and if you get along with step mum continue along those lines,it is her with the chip on her shoulder, ignore her,
I haven't spoken to my brother for 9 years after he crapped on my mum and dad, sadly both now gone, he didn't even come to the funeral, as they say "nowt so queer as folk" good luck,take care and don't worry,
you may not be asssertive but you are normal,,,, I would have to smack her in the earhole, but each to his own, Ray