I'm good at RV stuff, but need a bit more info.
Do you have a BLACK TANK (sewage storage tank) aboard the camper or is it a portapotty type? Of course, most systems which have sewage tanks also have Freshwater, and Gray tanks (which are for shower water, sink water, washer/dryer water, etc.) the black and gray valve is usually a single gate valve that leads to the two different tanks - basically a "Y" connector leading to a single 3 inch outlet.
Black/gray tanks usually have a 3 inch drain pipe with a slide type gate to release the sewage from the black tank system. If you have a gate walve type set up then you are well on your way to hooking up a more permanent system.
If you are really lucky, your sister has a cleanout plug in her sewage line (used for snaking any clogs) either at or near ground level in which you can run a flex hose to for dumping the sewage.
What I have is a 20 foot heavy duty 3 inch hose which looks like a giant slinky - It has a standard female connector on one end which hooks to the camper's black/gray outlet (you just align it and twist 1/4 turn) This screws into the slinky hose on a threaded end - hose clamps are a good idea, but usually not necessary.
Important point: you can leave the gate valve for the gray tank open all the time if you wish, but DON'T leave the black tank valve open: you need to build up HEIGHT in the sewage tank so that when you dump it, it will flow out of the tank properly. If you leave the tank valve open, you just create a dry stinky pile at the bottom of the tank which is almost impossible to clean.
the other end has a 90 degree fitting, one end screws into the hose, the other goes directly down into the ground level cleanout plug - which is nothing more than a pipe which leads to the sewer line that feeds out to the main town sewer system.
Installing a cleanout plug (what we would call a DUMP STATION) isn't hard if you know WHERE the existing sewerline is - of course, she may have a cesspool and I'm not familiar with the connectors for those.
If you know where a sewerline is, you dig down to it, cut out a section and slide a no-hub street tee (a 4 inch t-shaped fitting which has an angled tapered end to help the sewage flow toward the street. The no-hub has rubber bushings on one end to make up for the cut you made to slide the fitting in.
The easiest way to get this all done is to ask at Home Depot, Lowes or a local plumbing supply place - explain your project and they can show you the material I'm talking about.
I ran water, sewer and electric to my motorhome - it is ALWAYS functional during the warm months.