Fashion isn’t your mothers strong suit. You’re worried she’ll show up at your wedding wearing something tacky or inappropriate.
Seek the help of a third party. Ask her to join you for an afternoon of shopping, and use a personal shopper or sales associate to take the pressure of. She will be more likely to accept fashion advice from a professional. If she still ends up picking something completely hideous, keep your disappointment to yourself. In the end, a happy mother, wearing what makes her feel comfortable, is much more important than a great picture opportunity.