When the younger brother gets married, the elder sister-in-law can play many roles. What to do depends on the cultural customs at home and the wishes of the elder sister-in-law.

The following are some possible roles and responsibilities:

1. Assist in wedding planning: Brother and sister-in-law can assist in wedding planning, such as assisting in choosing wedding venue, music and catering.

2. Entertain guests: Sister-in-law can entertain guests on the wedding day, guide them to their seats, or guide them to the banquet after the wedding.

3. Serve as the best man and maid of honor: If the brother and sister-in-law are willing, they can also serve as the best man and maid of honor for the younger brother and provide support and help for the younger brother at the wedding.

4. Help prepare gifts: Brother and sister-in-law can help prepare wedding gifts for brother and sister-in-law, or provide suggestions and help for the family layout of newlyweds.

5. Participate in family celebrations: During their marriage, brothers and sisters-in-law can participate in family celebrations, such as wedding dinners and family trips.

Generally speaking, brother and sister-in-law can play a variety of roles in brother's wedding, helping brother and sister-in-law to successfully complete the wedding and send blessings to their new life.