But surely the sisters will have their turn when it comes ,I think this is an age thing ,in my experience 7 is about the age that children start to develop their own social life,interests etc.Up until that stage a lot of their friends are influenced by parents but now they start to take their first independent steps,this is healthy,this is what we are there for as parents to help to develop our children and to support them to be the adults that they want to be.
What a shame if Jaqueline du Pres' parents had said" no you can't learn the cello your sister isn't interested in tagging along to lessons or concerts",or Rebecca Adlington,"no you can't swim thats not fair on your siblings" .
When your nephew is sitting his GCSEs will he not be allowed to stay in and revise because its not fair on his sisters.
I do have a lot of sympathy,i was so relieved when ds1 finally gave up Rugby in some ways it did dominate but now he is 17 I sort of .look back on the camaraderie of those days with affection,it really is only a short stage in life.Now I spend sundays at chess tournaments with ds3!The key is balance ,one interest fine ,maybe two but somewhere you have to make sure they do still have family time .
On another note it does sound as if the interests of mother and sisters are trumping the sons.