There are many choices for you to choose from, such as couple bracelets, creative ornaments, couple life-like dolls, etc. You are very good sisters, so you naturally have a mutual understanding, and she will definitely be able to feel your blessing to her. Give her a pair of couple luminous pillows, which are both personalized and unique gifts. I believe she will like them.
The inner engraved crystals of the twelve constellations are also good. The colors are very bright, just like the gorgeous love that will never fade. It also symbolizes that their love is as pure and transparent as crystal, without any impurities. Consider a gift with a good meaning.
You can also give a cartoon sunshine jar, the kind that is hand-painted, and write your blessings on it. Or a crystal piano music box, you can also add their photos, which is unique with their own photos. She must be very touched.