Lan Wangji and Lan Xichen
If his baby brother really wanted it they could have a different placement. It would either be their beloved men between them or them between their beloved men. It would depend on what Wei Wuxian and Jiang Wanyin wanted to do. Depending on placement it would mean either the Lan brothers sat together or the others sat together.
"I do not have a preference either," He tells him with a bit of a smile. "Does Master Wei know anything about the holiday that was not confusing?" After all, Wei Wuxian had been there longer than all three of them.