From Friends S07E19, "The One With Ross and Monica's Cousin", it seemed that Monica got along with her cousin Cassie and I'm sure Rachel & Phoebe would know of her staying with Monica with the main characters being such close friends.
Why then wasn't she invited to the bridal shower? Would this be a plot hole because it's only until the end of the episode that Phoebe and Cassie are seen to share screen space possibly implying that Rachel & Phoebe never ran into her until after the shower.