Such pronouns "my" change with the gender they describe. And as brother is masculine and sister is feminine, they are "mój" and "moja" respectively.

"My child" would be "moje dziecko" (neuter)

"My parents" would be "moi rodzice" (masculine personal plural - includes at least one male human being)

"My sisters" would be "moje siostry" (not masculine-personal plural - for everything else)