Teaching my son to share?

Hughes • mommy of 4
My son is 10months an his brother is due in less then 2months an he doesn't like to share nothing! If daddy is laying on mommy he will smack daddy in the face an tell him my mom an same goes if mommy is laying on daddy. I don't want him being mean to his brother how do I teach him to share. Also he wasn't like this until his cousin moved in an his cousin is 3years old he don't share either an his mom let's him act like that her son has pushed my sons face down in the floor for playing with is ball! An if I say anything to her about it turns into a big deal an me an my husband end up fighting from it. I need help!!