Clear the air, tell her your all very upset about the fact that their are no pics of your sis and bro,and as they had the responsibility of managing that side of the wedding, they should have been a bit more thoughtful, dont let these things brew up, if you dont air your side of it now a lot of negativity will build up towards your MIL.
treat her with the utmost respest however do not be a doormat, that way she will know your a good DIl, however your not the type to turn a blind eye everytime.