My sister excels only only in English but also maths.

[Tan Elaine]

My sister excels not only in English but also maths.

My sister excels not only in English but also in maths.

Is 'in' needed?

Thanks.

 

[teechar]

I would keep it in there to avoid "my sister excels maths."
Alternatively, you could write:
My sister excels in not only English but also maths.