I think it largely depends on the school.
I know that Caltech asked about siblings in our son’s sophomore year and they gave a “discount” on our son’s tuition by ½, but tuition & fees, at that time, were about $72K.
When middle child was admitted to her med school, son was still in undergrad and Caltech again asked and we submitted the paperwork. We got ½ off again but it’s interesting that they didn’t ask us before son’s sophomore year-daughter’s senior year. It helped with the expenses and savings in the 529.
So, I really think you need to run those NPC’s at each school, where you will apply. (Note that Caltech is a private institution, with lots of dollars. I think they could afford to be generous. I don’t think that’s the case at any publics or with non-similar schools.)