Regarding that first part, it is absolutely false that she held back for Yukino's sake. It is quite literally the opposite with Yukino holding back for Yui's sake, while also dealing with very difficult emotional and mental insecurities ON TOP of pressing family issues.

If you pay attention in Season 3 (and also season 2 near the last couple episodes) you will notice Yui deliberately taking advantage of Yukino and her downtrodden state for her own benefit, and manipulating her too.

Both Yukino and Yui's side of that aren't presented as well as it should have been in season 3 due to a blatant adaptation bias, but even then you can still notice it if you pay attention.