My aunt breast fed me for over a year therefore my cousin is considered a sibling however is he non mahram? Do l need to cover in front of him as l consider him a brother. Also does the ruling refer to his sibling too and my siblings or only him and l because l was the only one that got breastfed.

Whoever was breastfed from the same woman will be mahram for each other. If your sibling didn’t breastfeed from your aunt then they would not be mahram to your cousin(s).