I have. I was at a bar for a band, a double date, me with my gf, then wife and now ex-wife,
And my sister with her boyfriend, now husband. It was a single use bathroom with a line a mile long, (divey kinda venue). Other dudes were doubling up with thier buddies. So my sisters bf and i looked at each other and said might as well, so my brother in law and i went in.... no big deal, he glanced at me, i glanced at him. Hes hung decent id say.
Days later i was talking to my sister and she told me her boyfriend made a comment to the effect of ur brothers girlfriend must walk funny after they fuck.... i was hanging a little heavy, from beers and needing to piss, but im basically a grower, mostly. so i cant imagine what he woulda said if he saw me hard. I thought it was funny till i realized my sister was talking to me abt my dick. Awkward.