I agree with Nat and Michaela (now how often does that happen?), that a man should not try to persuade his wife's female cousin to stopping wearing undergarments. Not only will the cousin think the guy is wierd and just wants to see her goodies, but the wife and other family members would likely think the same. If I were the husband, I wouldn't say a thing about it unless someone else brought it up, at which time I'd just say, "I don't care. I support my wife in whatever decision she makes." It would be very hard for anyone to take issue with such a politically-correct response.
However . . . . if the husband DOES want to see the cousin's goodies, he could use reverse-psychology (and only if someone else brings it up first) and say that he understands that the cousin is a conservative person and probably should keep wearing underwear. By saying this, the wife and cousin might both be reassured that he isn't looking at the cousin "that way." And, the cousin might then be spurred to show the guy (and herself) that he is wrong about her being too conservative to walk a bit on the wild side (I think a part of this bra-free stuff is rebeliousness).
Also, if there is one thing I've noticed about women . . . actually two things: 1) They want the approval of other women, . . and 2) They are most likely to do the opposite of a man's expressed preference for them. So, if the wife broaches the subject of "breast freedom" . . . and the husband says "I don't think so" . . . the next thing you know the cousin will be joining the wife in the undies-free lifestyle.