I'm kinda disappointed with his decision to go back. I really don't enjoy it when the mc becomes so strong that it becomes their responsibility to protect everyone. And while yes, the other characters have been giving it their all to help, it still feels like his presence stunts their growth because he always pulls through.
I think what the author should have done is allow the mc to drop out of the main plot a bit, have him go on some side journey, and focus on the growth and plot of the side characters. Let them overcome hardship until the MC's side plot inevitably ties him back into the fight near the end, with a new group of friends in tow to help.
And none of that would be an issue if that one sister hadn't made him promise not to get involved, because he only said he was leaving and that he'd still visit them. Basically no different than going on a journey.