When you're comparing a reseller account to a small VPS, bear in mind that the reseller server will be a much more powerful machine. The VPS is puny, but it's all yours. So if a site grows (or maybe gets a digg) on a reseller then that site may get suspended, but the same site getting the same traffic on a small VPS will most likely take down the entire VPS...
VPS is often sold as a step up from shared but really it's not - it's more of a step sideways. Either way you're sharing the resources of a larger machine and generally the share you get will be more or less proportional to what you pay.
Unless you know you need a VPS, you probably don't.
If it's just you and your sister you may not even need a reseller account - most shared hosting accounts allow you multiple domains. You only need the reseller if you want to give your sister separate control panel access.
And yes, you can host all the sites on one IP address.
Edit: didn't notice you mentioned SSL - you will need a unique IP for each SSL certificate you want to install.
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