I'd get annoyed, I'd tell them if they want to see Alex, (or William, or you) then they'll have to put up with him being fed!! If they can't deal with it, it's their problem, not yours
(if you were desperate to go though, could you offer to put a blanket over him feeding? not a chance with Ollie, if anything hides his arms or face, he pulls it off and shouts at me lol)
My MIL annoyed me christmas eve, about co-sleeping, but that's a tale for the other forums, and she said "oh, you'll be stopping bf soon with him being a year, won't you?'' yesterday, I just nodded and agreed, couldn't be bothered with the agro!!