Mc caved right a-fucking-way when his step sis said take the gift back, no spine bullshit
When he rejected the gift on behalf of the sister, why argue with her when she demands you do it anyways and pulls the "I redeem the favor you owe me" card to do it?
The only reason to argue, is if he is interested in playing the mindgames with her. Where he tries to figure out what she really means/wants, and the specific way she wants it.
Trying to argue with someone who is driven like that would either end with arguing in circles over nothing, or potentially even incur her anger over him being stubborn and having refused taking the gift back. She would feel patronized and like she didn't really earn it if he forcefully refused.
The only thing MC could've done differently/better is confirmed this is what she really wanted, and more clearly articulated his reason and belief that she was the one who deserved it. Maybe if he was particularly clever(he's not), he could convince her to have a contest with him over who gets the gift, so she could feel like she really earned it if she won.(clever being choosing a contest that isn't much effort on his end, and that she would win, since he felt she deserved it more having worked hard for it)
By all means complain about the no spine BS. I get it, it gets really annoying seeing it constantly in series.
It's just not really applicable to this situation for this character(who really doesn't care about it either way, and merely rejected it out of consideration for the step-sis)(if he really didn't want to go, then it would be a different story and I would agree)