Maid of Honor

You can but honestly it is just a title. I would say a lovely gift for her would be nice. I mean with the wedding so close I doubt you can change the programs unless you want to. I think a nice gift and maybe she can give a speech at your wedding?

I would suggest that you write her a really nice thank you note and give an extra gift to her to acknowledge all of her efforts, but I think titles should be given based on the relationship you have with the person, not how much work they did for your wedding. The MOH and bridesmaids duties are to show up on time to the wedding and to wear the specified attire. If you are closest to your sister and want to give her the MOH title, I say go for it, I don't think anyone should be offended by that!

Yea I would just get her a nicer or extra gift

I agree with PP. You could even, because it's your sister, tell her (in your thank you letter) you wished you had made her MOH from the beginning. But I wouldn't make it "official" or mention it to anyone else.

Not too late. The MOH is usually the person or persons who have known the bride longest, or are closest to them at wedding time. ( as opposed to back in school days, or previous job.) Tell everyone I did not want to set my sister off as different from the others,at first. But working together on the wedding, we have become closer than for a long time , and should be my maid of honor.
Do something to set off her attire from others, even if changing 2-3 larger blossoms of her bouquet to a different color, with other colored ribbons does it.

I agree with everyone that it’s mainly a title but to get her a nice gift!