If it's literally just he loves it and she doesn't then yes she is unreasonable. However, I suspect she simplified a more complex issue in the telling. Have you been to games and training, do you actually know how enthusiastic he is in when he is there, or how good, or whether the team is a positive environment, or how the other parents are wiut your sister? There may be all sorts of reasons it just isn't 'worth it's, but she doesn't need to go into it in detail in a message.
For example, my son did rugby. If anyone asked him he would always say he enjoyed it, but I stood twice a week watching as he wandered around the pitch making no effort to get anywhere near the ball, being actively scared of getting close to the other players, complaining he was cold etc. initially I was happy as it was cheap exercise ( he did jog around a bit) and social but actually he only really talked to the kids he was friends with in school anyway. As they got older and more serious and he wasn't improving or seeming more actually enthusiastic at all (as opposed to telling an auntie he liked it if asked), it seemed unfair on the team and a waste of several hours a week of family time to continue.
It also annoyed me that we usually didn't get times for the Sunday games, especially aways, until a couple of days before, so hard to plan family life around it. So I asked if he really wanted to and he 'didn't mind either way'. He stopped a year ago and hasn't mentioned it since or missed it at all. Perhaps your nephew has also said he's enjoyed being free from it over the summer? Unless you've had a detailed conversation I don't think you know enough to judge.
I agree with posters who say kids sports are often too demanding on family life. It seems really hard to get into sport if you don't start young (unless you are the naturally talented type who can catch up with your age group quickly) yet it's hard to try lots of sports young when so many very quickly ramp up the time commitment they expect, particularly team sports. And as much as I agree parents should do what they can to facilitate it, but realistically it is also important to earn money, look after your own wellbeing, treat other kids equally, ensure everyone eats nutritious things, prioritise education, spend time with family and the million other pressures that come with being a parent. Sometimes something has to give.