It is completely acceptable to have one person call and RSVP for several people. However, when there is a number AND an email address on the invitation instructing you as to which of the hosts will be able to accept your response, mentioning in passing to the other host (who lives in another state, and is not set up to handle replies) that you will be attending an event with two other people really does not count as a reply.
Especially when you would TOTALLY never let someone hear the end of it if someone did that to you.
This is almost worse than not replying at all.
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Please note that this is for a large party, and this is extended family. This was not my mom mentioning something to my sister. And either way, it comes from someone who has been known to make up "etiquette" and then hold failures to conform to the made-up rules against people, sometimes, seriously, for years.