For a family unit of 4, an A-frame camper would be too small. Jayco has started making them the last few years and the various units of Forest River has been also. But both only make a 12 ft box and at most they sleep 3. Here are some pix of mine (a 14 ft box) which has a sofa bed in the rear and a dinette which folds down to sleep a third. As I said above, it's good for me and DW entering our empty nester phase of life. A-frames are also referred to as "folding trailers" to lump them in with regular pop-ups.
I had a popup for 10 years (bought it new) that was great for our family of 5 but was work to set up and take down and now that all my helpers have grown up and flown the coop (one left) and it's ME doing all the work, we changed. It was a Jayco with a slide out dinette for extra room and king/queen slide out bunks. Weighed between 3,000-3,500# which would be too heavy for your sister's Odyssey.
I would suggest (after verifying their tow limits on the Odyssey), they look at popups with a 10 ft box and no slide outs. I would recommend Jayco but there are other units out there. Used units are all beyond warranty and then it's a question of what condition it's in. Popups are good for sleeping up off the ground (we started that way too) with some amenities but it is still "camping". One down side to me was popups are 3 season campers where with my Aliner I can camp year round quite easily. A popup can go through the winter/real cold (4th season) but again it takes extra work.
Bama Ed