How do you stop stress my mom?

Look, my mom is kinda old and has a baby in her stomach but me and my step sister keep on fighting and making my mom stresses! And i don't want to give her stress! ( My parents are divorced and im living with a step dad and my twin brother and little brother is living with my dad and my mom re-married and i have a step sister and my mom had sx and it made her have a baby! And i don't want to give her stress!) What should i do please answers only! Please help!