Now that Sinner has won Rotterdam, he has a realistic chance to become no.2 after Indian Wells.
First, if Sinner wants to overtakes Alcaraz in the rankings, then he NEEDS to win Indian Wells. This is priority number 1. If he does not win IW, then he can't be number 2 in the world after IW.
As for Alcaraz, Rio Open is important to him not only for his confidence, but also for his ranking points. If he does not win Rio or reach the final, then he will drop points going to IW. And if he drops points before IW, then Sinner can overtake him in the rankings assuming Sinner wins IW (stated in the 1st point), of course.