It makes sense -- uncles and cousins live far away, so you don't see them often, and the "mystery" necessary for sexual desire is retained, but there's a familial intimacy that makes sausage sightings much more likely than with strangers.
Here are my top 20 family-member sausage sightings, gropes, and grabs.
My Family
Ken, my brother. Lots of times.
Terry, my sister's husband. A bit homophobic, but still, I got a glimpse in the locker room when we stripped down to work out together.
Dad's Family, the Davises
Cousin Joe. My very first sausage sighting, when I was 7 1/2 years old and went to the bathroom late at night, to see my older cousin there, washing off in the sink.
Cousin George. From South Carolina, exactly my age. When I went to visit him at age 10, we took a bath together, and slept in the same bed, naked: "only fools wear pajamas."
Uncle George. His father. When we went swimming, we all changed clothes in the same room, and I got a good view of his cut Mortadella+ hanging down.
Cousin Phil. One Thanksgiving evening my brother and I had to share a room with my older cousin. I got not only a sausage sighting, but a sausage grope and fondle.
Cousin Donnie. Actually my third or fourth cousin, from Canada. Grandma Davis brought us out to visit one summer. I got a good view in a bathhouse at the beach.
The full list, with nude photos and sexual content, is on Tales of West Hollywood.