Even a half decent tailor for a wedding will account for that. If they can't fit you into a vest, they'll have dickies that match the rest of the wedding party's vests. Also, note the lapels on Bob's and Chippa's suits don't really match. I think this was a piecemeal affair, and Bob genuinely doesn't own anything decent, and couldn't be arsed to buy a fucking dress shirt for his own sister's wedding.
Further, and I can't believe I never noticed this before, he's clearly wearing the same polo shirt in the infamous hunched-over-his-phone-at-a-wedding-reception picture. It matches perfectly.