now rc....take another angle....what if it was your younger brother having to train you? that's one of my situations...the 5 year old can play better than the 10 year old...but the 5 year old also reads better, is better in math, etc..5 year old is gifted.... so the 10 year old feels dumb compared to her sister 5 years younger..
speaking as a teacher (maths), I'd say **one possible** issue happening here is:
The younger one was born when the older was 5. The baby gets the attention so the older one
regresses...to get attention. Attention seeking behaviour includes actually feeling dumb, it can be
interesting from a theoretical perspective but heartbreaking to watch.
Then of course:
The younger one is the adored baby who therefore has the confidence to "just go for it" and
therefore succeeds. The older one is inhibited, and now at 10 years old is starting to want to
carve out an own identity. This could involve, in order to be different from the sister, becoming
underacheiving even though native talent is there.
Anyway, the cure is to try to carve out a separate musical identity for the older one, to make
(her own) comparison difficult...so using different tutor books, different exam boards etc is a start.
Fortunately piano music is such a hugely wide field this should not be difficult. You only
need to build confidence and then the real talents and interests come out naturally.
At some point, each child needs realise they have to do as well as they can for themselves
regardless of what anybody else does. This is a very hard lesson for a 10 year old,
but it can be made easier for her.
Good luck!