Assuming she's British or American, I think it's very odd she would do this. I've always been aware it's an unwritten rule for women not to wear a white or cream outfit to a wedding. So for a woman to choose to do that, you have to wonder what they were trying to achieve.
At the same time, in the photos you have where she's there, would anyone have a hard time working out who the bride was? Because I'm guessing not. I would try and hold on to that perspective so that you don't fall out of love with your wedding photos. If you really can't stand them, an alternative is to get her outfit coloured by a photo editor. (But I would expect that to cause drama if she ever sees them or gets told about it by someone else. It also means it's a less "true" depiction of the day, which you or DH might not really want.)
But I certainly think it would be wise to be a little wary of SiL and not take her friendship or civility at face value.