I do not think it is rude at all PROVIDED, as you say, you are doing in a familiar circle. On the contrary, I think it is a proof of affection towards them AS LONG AS you do it, as you say, in view of your cousin.
When we gather with family members it is nice to show a fair amount of informality and kindness.
Behaving too formally may tend to show a lack of appreciation for the members of that family or a degree of arrogance on your side.

Watch the dates on old threads. The advice is still applicable here, but the post is from 14 months ago, meaning the issue is long since resolved. Doesn't mean don't post... just a good I idea to direct energy toward posters needing help presently.