obvioulsly jealousy on the part of the 10 year old as well as insecurity. Potentially the parents could favour their adopted child over the fostered one even unconciously. Hope(or fear) by the fostered child that he/she may also become adopted by the couple (depending on the age of the fostered child which you don't mention) and then just conflict between the two children if they dont get on for whatever reasns, age, sex, background, interests.Finally the parents may have difficulty handing back the fostered child when the time comes, as might the ten year old if they have got on