Generally, OurSect Buddhists (and just so you know, Buddhas are not deities; they are enlightened teacher souls who help us also become enlightened. The misconception lies in translation) don't attend services at other temples/churches. We may go to weddings, funerals and the like, and for close family/friends, and sometimes we do. Sometimes we decline, and send a note and/or a gift. Sometimes we are invited to the reception/wake only, and we go. We can and do memorial services at our temple (they are no big social deal in that there are no guests, no wake, etc., unless the decedent is a member, but spiritually very meaningful to us.)
We would decline this invitation as it is not a major life event for Nephew, and say we would love to hear him sing another time, perhaps at our annual Solstice/New Year's party? So much of "Christmas" stems from older religious practices (Sol Invictus, Mithras, Druids. Etc.) that it is very easy to confuse the "secular" Solstice based Christmas with the Christian Christmas, and many people don't get that some do the Pagan parts happily but won't do the Christian parts. It confuses them, and that is part of what OP is running into, I think. Since none of it is Buddhist, we don't do any of it. Don't worry. We still have parties and presents. We just do it in celebration of the New Year, in a tradition that predates even Sol Invictus. OP, stick to your guns and suggest an option.