Well, technically no one but the bride should wear white to a wedding. But the reasoning behind that “rule” is that a woman who wears white to another woman’s wedding is trying to upstage her. In your case, it sounds like it’s highly unlikely that your mum is doing that. As long as her dress isn’t lacy and frilly — meaning, as long as it doesn’t resemble your dress — it’s okay for her to wear it.
Of course, it’s okay to make your feelings known, too. You don’t have to get into an argument — just say something like, “Mum, you know how much I appreciate all the help you’ve given me with wedding planning, and I know my wedding will be a big day for you, too. But I’m a little uncomfortable with the idea of you wearing white.” This may or may not change her mind, and she might get defensive, but you have a right to say it. If she tries to draw you into an argument, just refuse — say, “Look, I just wanted to tell you how I feel, that’s all.” In the end, don’t worry too much about this. Either way, you will be the centre of attention on your big day, and your mum will be glowing, too.
by The Knot