I will explain this using a slokam from Sri Vishnu Sahasranamam.
Upendro vamana: pramsur
In the Vamana Avataram, the fifth Avataram of Sriman Narayana, he incarnated as the son of Aditi, the mother of Devendra and the other Devas, so in that case he is the brother of Indra. Though he was technically the younger brother of ...
Sandhi Vichheda of Upendra is Upa + Indra. Here the word Upa have many meanings but two meanings "younger brother" and "above" are relevant here.
Upendra = Upa + Indra = Younger brother of Indra
The youngest son of Aditi is Lord Vishnu's incarnation Vamana (Vishnu). As Vamana is younger brother of Indra, he is also known as Upendra. Due to this ...