Hi- I’ve had the same foster brother for 11 years now - and I too find it very hard to like a child that puts so much stress and pressure on my parents - to the point where my dad is sacrificing his own health because of the stress this child- who is now 11 and a half causes.
My advice is- talk to someone- vent your anger somewhere, because deep down , your feelings matter and need to be recognised. You’ll be surprised how your own emotions will change towards your foster brother