Your resort/DVC reservation does not carry over any requirements as far as the number of people on the dining reservations. So, even with 4 confirmed in your room/unit, you may make reservations for however many you need

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Do note that if your family and your sister's family are splitting up for meals and eating at the same time in different locations, either she's going to have to make her own while you make your own, or you're going to have to call. It will not allow you to book 2 different ADRs within a certain timeframe of each other, even if you put totally different information on the ADR (it's all tied into your login/session on the site). You MAY be able to sign in under a different account and make them (I'm pretty sure this would work, but I'm not sure if this is actually controlled by login only or if your web session comes into play as well)