According to the traditional wedding custom in China, on the day of elder sister's wedding, the bride and groom usually only need to offer tea to their elders to show their respect and gratitude. Sister, brother-in-law and sister's children don't need tea ceremony.
However, this may also vary according to regional and family customs. In some places or families, children of younger sisters, brothers-in-law and younger sisters may participate in tea worship to show family unity and affection. If your family has such a tradition, your sister, brother-in-law, and sister's children can all attend the tea ceremony and salute and offer tea according to the general process.
In short, the specific participants can be determined according to family traditions and personal wishes. Before the wedding, it is best to discuss with your family to understand their customs and expectations so as to make appropriate arrangements.