Without looking anything up, I did not find that phrase confusing .. "From ancient grudge break to new mutiny" My immediate read is that the two houses have

Without looking anything up, I did not find that phrase confusing .. "From ancient grudge break to new mutiny" My immediate read is that the two houses have an ancient grudge, and there will be a change in status involving mutiny, in this case the love between Romeo and Juliette(their houses are enemies so you could see this as a form of mutiny). Who can say if this is what Shakespeare meant, but the phrase didn't seem hard to follow at all, I didn't have to think about it.