I pull a 7200# camper with my 2003 Expy 4x4. 5.4L, HD towing, 3.73 axles, four speed trans, rated at 8200# towing capacity. I don't boon-dock so don't carry a tank full of water, but I'm still right at or a bit over the recommended tow capacity. Towing capacity doesn't include any passengers, luggage or equipment in the vehicle or camper. It tows okay. Not going anywhere in a hurry, but it WILL tow the trailer at 70 mph in relatively flat, modest hill terrain. Will lose 5-10 mph going up hills depending on how long and steep. In the mountains it will slow to 40-45 mph in third gear going up inclines. It won't drop to second unless I really press on the gas pedal. Makes more noise and burns more gas, doesn't really speed up much (if any). I don't go in the mountains often, and I rarely drive much over 60 mph. Power to increase speed over 65 mph goes up exponentially due to aerodynamic drag according to tests conducted in the 70s. My Expy, especially with camper in tow, is about as aerodynamic as a early 70s car... maybe slightly more? So I drive 60-62/3 at the most. Gets 8-9 mpg with that load, 16-17 any other time.
A six speed trans would be slightly better towing, but when I bought the 2015 about three years ago it was in better condition than newer models and had just under 100K on the clock! Had been babied and garage kept it's whole life, and the price was right!! I was looking for a 2011-2013 so I could get the six speed, but mileage on all I saw was 125-150K+, price was about $4-6K more, and they looked like 10 year old vehicles. My 03 STILL doesn't look 10 years old, much less 18!!
The rating for the 2015 with Eco-Boost (only engine offered) is 6000# for the 4x2 with 3.15 axle, 9300# with 3.73 gears. Bear in mind that the tow weight doesn't include load in vehicle or trailer. There is some wiggle room, but a good rule of thumb is to allow 1000# for load as a minimum, more if you know for sure you're carrying more (like the water in a tank if boon docking, and weight of generator, etc.) It's just me, my wife, and 50# dog, so I'm pretty good with that, but if you have a couple kids and take a lot of "stuff" with you, you'll want to subtract more -- 1500# might be a better number. So if it's just your sister and hubby, maybe a dog, she will be fine pulling a 5000# camper as is. It won't be something like my three slide 30 footer, more like a Coleman Clipper 21FQS (4695#, but it's a good camper for a couple, but no slides
https://coachmenrv.com/travel-trailers/clipper-ultra-lite/). Look at the EMPTY weight, or GVWR (or UVW) listed weight. That should be no more than 5000# for the 3.15 geared 2015 Expy. Limits you to trailers with no slide or a small slide in the 21-24' range. Something like a Forest River Cruise Lite West T197SS (4,113# empty). The larger campers on this site would work:
https://www.thewaywardhome.com/travel-trailers-under-6000-lbs/.
Just know what the specs are before looking and look at the sticker on the side with the GVWR BEFORE you even look inside!! No point in looking at something you can't tow without buying another vehicle. You may as well find out what that axle upgrade is going to cost you before looking too -- the larger they are the nicer!