So, basically, you are the voice of reason in your family?
An assault weapon is a weapon that is capable of selective fire, which means that it can fire in single shot mode (semi-automatic, where one trigger squeeze equals one shot) or a burst/multi-shot mode (full-auto, where the weapon fires automatically as long as the trigger is depressed). Fully automatic weapons are already banned, so what your sister really wants to ban is any repeating weapon, i.e., any weapon that can hold more than one round and which can be fired without having to manually engage the action. There are currently over 80 million households in the United States with gun owners (officially, but I believe the real number to be substantially higher). An outright ban would transform roughly half the country into criminals, and a signficant percentage would not only resist confiscation through subterfuge (hiding weapons, fake sales, etc.,), but would actually fight back with those guns if someone showed up to take them. Even if nobody fired a shot, just the sheer volume of searches and warrants would become a civil liberties nightmare. If it turned violent, it would be on a scale not seen here since the Civil War (in fact, it pretty much would be a civil war). Most of the police and military would oppose the ban and if it came to open defiance, would side with the gun owners, which would severely impede confiscation attempts, and turn large swathes of the country into no-go zones for law enforcement. Think of the Prohibition era, when police fought bootleggers and home-brewers in every town in America, and you get a vague idea of the type of issues involved, but the scale would be an order of magnitude greater.
In order to carry out your sister's attack on the Second Amendment, we'd have to void the First, Fourth, Fifth, Ninth and Tenth Amendments, not to mention eliminating all manner of other legal protections. For someone who probably considers the Patriot Act unacceptably invasive, she's sure advocating a lot of civil liberties violations.