I'm trying to find a manga in which there are two popular brothers with a plain and unpopular sister who don't look even a bit like them. One brother is brunette while other is blonde. There mom is a nurse with a very bold personality. There's also a rival of the brothers who to take revenge dates the protoganist but fails. Later it is revealed the main characters are not related to each other and one of the hero is actually stepbrother of the rival. The protoganist realizes that she loves him after he left for some duration.

it's gotta be Sonnan ja nee yo

Yes it is!! thank you 🙂