"свой" is used when you talking about another person's feelings/action to his family members. he loves his sister (он любит свою сестру) but when you talking about your or somebody else feelings/action/discussion to family members of this person "свой" shouldn't be used. we saw his sister (мы видели его сестру) or when you discussing some facts (without any relation) his sister's name is Alena (его сестру зовут Алена) my mom is a teacher (моя мама - учительница) when discussing your family members "свой" shouldn't be used to avoid mistakes. sorry for my bad english, hope this answer will be helpful