We could go to the beach or to your sister's house, and if all else fails we can always find something to do at home.

Explanation

- The sentence contains the phrase "if all else fails", which is a common idiomatic expression meaning "if everything else doesn't work out". However, in this context, it would be more grammatically correct to use "if everything else fails" instead of "if all else fails".- The word "all" is not necessary in this phrase, and "everything else" is a more precise and clear way to express the idea.- A more correct sentence would be: "We could go to the beach or to your sister's house, and if everything else fails, we can always find something to do at home."