Knitting

You can definitely do this. Before you choose a final pattern and yarn, I'd definitely coordinate with your sister to make sure she likes the choices, and they match her dress/theme. Matching it to her dress choice is actually an opportunity to make the item even more unique and special for her...like adding some bead edging accents that match a sparkle or pearl accent on her dress.

For the knitting itself, a few pieces of advice:

-Do a small test piece with the yarn/needles you're planning to use. This will teach you how to start a pi shawl (fiddly if your first time), if you want more slippery/sticky/blunt/pointy needles, and let you see how it blocks out. This can be your memory doily for your sister's big day, or perhaps give it to your mother (if you're close) or to the bride as an additional memory.

-Lifelines, lifelines, lifelines. I'm pretty cherry about dropping down and fixing complex patterns, but lace is extra challenging because of the fine yarn and the fact that the stitches are often harder to read before blocking.

-I love Elizabeth Zimmerman's Pi shawls. But be aware that the outer rounds take FOREVER. So when you've got half the radius done, you are not halfway through. You still have lots and lots (and lots) to go. Just be aware of that from a time management perspective.

Good luck, and I hope your sister and you both love the pattern and yarn that you choose!