A reasonable person is going to accept "she has a graduation and a wedding to go to." An unreasonable one is going, next time, to say "but she skipped Sue's shower for those things for her family, she has to come to Livia's!" and/or try to find ways that the person could go to the shower as well as the pre-existing events.
With reasonable people, I say things like "I'd love to, but I'm busy, maybe next time" or "I'm sorry, I can't come to that, but are you free for dinner sometime soon?" (if it's someone I actually want to see). But not everyone is reasonable; most of the people posting here have an idea, by now, of which of their relatives, friends, or acquaintances will take "sorry, I can't" as the opening statement in a negotiation of how you are, really, no matter what, going to do what they asked for.