Is it bad manners to bring your boyfriend to a party that you were invited to?

I was supposed to go with my sister, who was invited, but she doesn't want to go. Is it rude to ask the girl if I could bring my boyfriend? I've heard it's rude, but I don't want to go by myself, I don't know anyone else that's been invited.
Is it bad manners to bring your boyfriend to a party that you were invited to?
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