Maybe she isn't doing it on purpose.
I know I am very clingy with my boyfriend, and if I weren't vastly uncomfortable acting that way around people I might do the same thing.
Maybe she just doesn't think of it as a rude thing to do, even though a lot of us would? (I certainly do.)
I would try mentioning it nicely "Hey, it's not a huge deal, but you guys' PDA makes me feel a little awkward."
that way you're not fighting, you're not like.. demanding she stop showing her husband affection, but you ARE letting her know it is hurting your feelings, so if she continues from there, then you know it's intentionally insensitive, and you can take further steps if need be.