Not all that wierd. Harry married Ron's sister, then Ron married Hermione. A little weird to "look-back" on, but not a lot. Its not like brothers-in-law(-in-law!) are taboo!
its not like "oh, I'm your grandpa and your uncle, and your grand-uncle, oh and your daughter to" type thing... but its still wierd.
im sure someone is writing a very kinky fan fiction about all of them in an orgy right now.
Probably 50 or 60 someones.
your estimate is way too small... think bigger, with strippers.