17yo living with grandparents

A 17 year old (in non-advanced FTE) is estranged from her parents but living with her grandparents. I assume the grandparents will be treated as ‘acting on behalf of’ the parents and therefore should really be claiming CB/CTC for young person. If they don’t want to do that (for whatever reason) is there any chance of the young person being able to claim IS? My guess is that living with grandparents always rules that out?
Don’t deal with younger people very often!!

I agree. Grandparents will be viewed as in loco parentis so IS on estrangement grounds would almost certainly be refused on that basis. Grandparents should claim CB/CTC.