Stannis Baratheon is a fictional character in the "A Song of Ice and Fire" series of epic fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones". He is the second son of Steffon Baratheon, the lord of Storm's End, and his wife Lady Cassana Estermont, and brother to Robert and Renly.

Can we draw the following conclusion?
Stannis Baratheon is the older brother to Robert and Renly.
It's impossible to say