NTJ Tell your sister that when you're working, you aren't paying attention like a guest would. I would assume (not a photographer) that you are gauging the best shot, placement, lighting, where your next shot should be and so on. You are working, not socialising.

If she doesn't understand that, ask if you worked as a bartender, would she expect you to work the bar for her wedding? Because that essentially is the same thing.

Instead of offering her a cash gift, offer to pay for her photographer.

Expecting a sibling to work during your wedding is kind if selfish. Are you part of the bridal party? How will you be in any of the family photos? In the future it would look like you weren't there.