Living in that area maybe a Muslim!
I know that many Muslim families won't allow a male tutor for a female child and vice versa so I am guessing that this is a situation like that.
This request would then be more cultural than sexist.
Alternatively the sister is in Year 8 - just reaching 'that' age and they are asking on a forum where young males are more likely to read the site than older more mature males. To a Year 8 girls the older males would be less attractive ones. In other words they are asking for a tutor who is likely to get an academic response from the girl than a romantic attachment.
I agree the request looks sexist but there are alternative logical explanations as well. Sexist, racism, agism etc are easily made allegations but sometimes there are logical and acceptable reasons behind the request.
If you need her to be tutured in History I would be pleased to talk to you as I live nearby and am female but I couldn't do either Maths or Science.