While everyone is counting down to Pippa Middleton’s wedding this weekend, there’s been talk about exactly what role her royal sister will play. With gossip mostly centering around possible wedding gown designer Giles Deacon, and the fact that guests have been asked to change into different outfits during the event, no one has really questioned how the younger Middleton will fit her sister into the wedding. And the reasoning has to do with royal protocol.

Pippa Middleton

According to tradition, a bride-to-be’s wedding party should only consist of unmarried women. While we sometimes see Matrons of Honor at weddings, those who are connected to the royal family are expected to maintain traditional protocol, which means that Kate can’t preside over her sister at the wedding.

According to one royal expert, Kate likely won’t be involved in the wedding at all outside of being a guest. “Kate would upstage her sister,” Judy Wade said in an interview with People last year. “It’s a tricky situation for Pippa. She would want her sister by her side, as who else would she trust to make things go well? But if your sister is a future queen, she is going to upstage the bride and draw attention.”

Even though Kate might not want to upstage her sister at this weekend’s wedding, another guest might. Word got out last week that Prince Harry’s girlfriend, Meghan Markle, would be in attendance as the Prince’s date. Who knows, maybe she’ll catch the bouquet.

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(h/t E! Online and Redbook; photos via Pascal Le Segretain + Ian Gavan/Getty)