Personally I would have the elder sibling tested. Higher IQ is not always better, there is an optimal intelligence range.
- This
article mentions 125-155 as being the "socially optimal range." Note that was in 1926 when the ranges were broader.
- This
article mentions 120-125 as an optimal IQ range for leadership. Related thread
here.
I would help her understand where her IQ is on the
IQ curve, and the relative
rarity of her IQ score.
I would help her work on addressing any self-doubt or lack of confidence about her intelligence with facts and logic, rather than relying solely on emotions and opinions.
People have different physical characteristics, including their brains. There is an art to accepting ourselves and also accepting others.
It is possible than any child may have both high IQ and a learning disability or learning difference which may tend to mask the high IQ. Compensating for this second exceptionality (2e) may divert a lot of a person's time/effort. For example, studying and/or completing homework may take longer.