The woman is clearly bonkers!
I hate to say it but I think she's going to "whip one out" albeit discreetly, just to prove a point.
You have a few options open to you.
1) politely point out that she was right, it would be illegal to ask her not to, blah, blah, blah but out of love and respect for her sister could she abstain from doing it, express before hand, or not bring the dear little darling?
2) uninvited her? Me thinks not really practical.
3) if you've tried suggestion one then have her sit on a different table, and tell her exactly why... You after all want all eyes on your new wife.
4) bowl her over with flattery by saying something like you know everyone will be comparing how beautiful the two sisters are and you wouldn't want to outshine your sister (maybe with this one not even mentioning the sprog