Aruni served as charioteer of Surya, the sun-god. Aruni's female form and Surya gave birth to Sugriva. While Aruni's female form and Indra gave birth to Vali. Both the children were left in the care of Gautama and Ahalya.
(There are other legends that say that they were born to Ahalyai through Indra and Sun (in a story that was mentioned earlier) and Gautama cursed them to turn as monkeys when he learnt they were not his progeny.)
In any case, the fact that they are half-sibling doesn't change. They were given by Indra to Riksha - the childless monkey king of Kishkinta.