The sister is sweet, but who in the hell chooses a YELLOW dress for a winter wonderland theme? I think the sister is just uncomfortable with the whole theme, but didn’t quite understand that it’s the BRIDE’s wedding day. She gets to choose. So sorry.
If she wants them to carry a muff, they need to dress like something out of “Anna Karenina” or “Dr Zhivago”. Cold weather gear like a muff looks incompatible with lightweight dresses, especially with a winter theme.