Everyone has brothers and sisters to take care of them when they were young. This is a very beautiful memory. As we grow up, maybe the exchanges will decrease. You must send your blessings to your cousin's wedding. As for the gift, I can give you some suggestions:
First, give him a blessing gift. You can choose a crystal ornament. Swarovski has a classic snowflake crystal pendant that can be placed at home. It is a very good home decoration and also has your wishes.
Second, give him a gift he likes. Although it is not a birthday, it is also good to give a private small gift on such an occasion. Giving him a small gift he likes will also surprise him.
Third, give him a creative gift. A crystal music box is good. I believe that the exquisite small gift will also be liked by my sister-in-law. Of course, you can also give them a real-life doll of their wedding together, leaving this happy moment to them forever, which will definitely make them very happy.