Hi, Mom:
Let me understand this situation:
Your mom bought your daugter, her grand daughter, a jr. bridesmaid;s dress for your a) sister's wedding in November.
You were not there because your mom and your daughter went with your b)sister for her wedding dress fitting because she is getting married in May 2013.
You feel upset because she didn't say anything to you about it and you felt excluded from the experience.
From what I read: You didn't go because your mom and daughter were going with sister (b) for her fitting so you didn't think about maybe your daughter would be able to buy a dress for the November wedding while sister (b) got fiitted.
It sounds like you are blaming your mom when you failed to go with them. After all, sister (b) didn't have your company to share her experience.
You failed to be supportive of your sister and this is your consequence
for failing to support your sister. If you had of supported your sister, then you would have been part of the decision making process for your daughter's jr. bridesmaid dress.
Good luck.
D.
.