They weren't that much weaker Natsumi, star value alone doesn't count for everything. Some of their players had higher tactics, reasonable primary stat levels. Keeper isn't far off yours. They also had home advantage and you don;t know what advanced tactics they used or how they affected the game. You also matched them up 442 diamond, which away from home normally means you need a much stronger team.
We've all been frustrated at some point but it would be very boring if only the best teams won ALL of the time.
If it was simply the strongest team on stats that won every game, we all might as well give up because it would be virtually impossible to catch the longstanding teams who are already far advanced in the arms race of facilities upgrades.
Just look at the top of the real Premier League in England this season. It's not all the best teams that are up there.
I lost to seemingly worse teams than myself last year, with a couple of exceptions, but also beat teams that seemed stronger.
If you really want to play a game where tactics are transparent and it is always the best team that wins, then maybe this isn't the game for you, but you've seemed to enjoy the banter and fun as well, so I hope that's not the case.
So I'd advise that you don't leave it looking like you are throwing your toys out of the pram. That would be a shame. There's a long way to go yet and you may well still be up there. Don't make hasty decisions like your sister did, or you may regret it.