As Salaamu Aalaykum,
Isn't it interesting that children don't see other siblings with the labels "adopted, fostered or biological?" If they live with each other day in and day out, they fight and play to each other they are simply siblings. Its the adults who feel the need to categorize the relationship.
I don't think my toddler sees his sister as his 'biological sister' or loves her BECAUSE she is his biological sibling but simply as HIS sister. The one who steals his toys and occasionally his food but his sister and playmate all the same.