How to politely decline wearing future MIL's Celtic jewelry at wedding?

She has this whole collection of Irish jewelry she's offered to lend me for the wedding. Celtic pieces that have been in their family for generations. It's a sweet gesture but I already have my own grandmother's jewelry I'm planning to wear and I don't want to be weighed down with multiple heritage pieces that aren't mine. How do I say no without seeming ungrateful or like I'm rejecting her family's culture? She keeps mentioning specific pieces and which family members wore them at their weddings and it's getting harder to deflect without being direct. My fiancé thinks I should just wear one piece to make her happy but I don't want to start our marriage by doing things just to appease his mother. This feels like a small thing but also like it's setting a precedent for how much influence she'll have over our decisions. Am I being unreasonable?