well i have 3 cats. ,mamma and two kittens, girls. mamma got pregnant again before we got her completely inside and she ignored her two girls due to that. the girls became best buds and were "joined at the hip" for awhile. then mamma had six more kittens. after they were weaned and she lost interest, the girls "unjoined" themselves. i found homes for 3 of the second litter and gave 3 to a place that found homes for them.
now mamma and her two girls chase each other, wash each other, hiss at each other, slap at each other, etc.
they are just 3 cats living in the same house. not mamma and dtrs.
you two are just 2 cats living in the same house. scold your mamma or son when either one is mean to the other. they should learn by the tone of your voice what is excepted or not.
good luck.