Sooooo I remember watching this anime a long time ago of two step-brothers with the younger one crushing after the older one.

These were the scenes I remember:

(1) It was in the first episode (I think) where the younger brother wakes up the older brother and pulls down his pants, and then there was this big rainbow that popped up with typical colorful images.

(2) The older brother goes to his high-school exam and then when he gets his results, the younger brother gets sad about something and then he leaves the house.

I'm pretty sure it's not Super Lovers, but it was a really good anime and I hope someone can tell me what it is.

This seems quite interesting since I’ve not heard of any BL anime about brothers other than super lovers or love stage.
I will be on the lookout tho :>
Try going through this list, I think it has most BL titles on it. (If you haven’t already 🙂 )
https://myanimelist.net/anime/genre/33/Yaoi
https://myanimelist.net/anime/genre/28/Shounen_Ai