It depends on your relationship
with your sister and how much you want to have this hair change.
I have two examples of similar situations. In the first, I had been growing out my hair for my own wedding, and mentioned to my best friend from high school that I couldn't wait til I got back from the honeymoon to chop it off again. She said "Um, well you're going to have to wait, because I want everyone to have matching updos for my wedding." I was surprised, but given our long-standing relationship
and the fact that her wedding was only one month after mine, I said I'd wait. I chopped it off the day after HER wedding instead
was more important to me than the hair. I did think it was unreasonable of her to ask, but *I* decided I'd rather go along and get along.
Then, about three months ago, I had another friend getting married. I was planning on dying my hair dark, but then remembered the first story and thought that I should check with my friend first. She said, "Why would I care what color your hair is? I love you, not your hair color."
It would be wonderful if your sister had been more like my second friend. But the reality is that she's not, and it seems like doing this will deliberately upset her. Only you can decide which is more worthwhile to you, the relationship
with your sister (and, based on your earlier threads, it seems like she's one to hold a grudge) or your hair for a period of time.
I, personally, would choose the relationship
with the sister.