Sister-in-law forces bride-to-be to borrow her blazer for wedding ceremony, doesn't understand why she refuses

Does "something borrowed" mean you have to wear your sister-in-law's blazer?

If you're getting married, you're probably thinking pretty hard about what you're going to wear. It's most likely the biggest night of your life fashion-wise, so you're putting a lot of thought into what duds you're dusting off. Wedding attire is expensive and specialized, and there is a whole industry that revolves around it. If you're wearing a dress, you're probably spending a lot of time and effort to make sure that it's exactly what you want. You don't want a last-minute addition throwing everything out of whack. The bride in this story surely didn't, and had a hard time explaining that to her family. 

Tradition is important, but only if you think the tradition adds value to your life. If it doesn't, then you should examine whether or not you continue on with it. When you marry into someone else's family, the expectation is that you're marrying into their traditions. This can sometimes cause some issues, though, when you have clashing ideas about what a certain milestone or event should look like. The two families in this story did, when a wedding was afoot, and the bride was receiving all sorts of unsolicited advice about what she should do on her wedding day. You don't want to immediately shut down the traditions of your new family, but you also probably don't want to wear a blazer on your wedding day as the bride. That's a pretty big ask.