HAHA! I think you're not alone ... my friend, though not married yet, she told me that her bf's bro's gf also like that... even she purposely say "HELLO" very loud she still can ignore!!!!!!
as for me, last time when I smile to my sister In law she also lik that then i often complain to my hubby but he dont believe ... he say i smile/hello not obvious so his sis cant see but i think it's a BASIC courtesy to smile/hello to anyone you know even I didnt do it to her... then slowly i dont really care but now dont know y better alrdy at least will smile to each other ..
think of a bright side, at least your hubby know that his sis is rude... if he doesn't know and accuse you of accusing his sis, that'll be very bad right??