So, to paraphrase, you'll have 2 adults 50 percent of the time. The other trips will include up to 3 additional adults (sister, father and mother) and up to 4 kids.
There is no way you're going to find something that will work for all that, so I'd suggest splitting it up.
First, buy a tent for the kids. When 2-4 come, they all go in the tent. If just one of them comes, then you fit them in the TT or van.
Second, get your sister to buy (yours or a different one) a van. This will provide extra sleeping space and something to haul the family when everyone is coming. Nothing special - just a van with easily removable seats so she can haul people and then get lots of floor space to lay out an air mattress and sleeping bags.
Third, utilize the outdoor space. You're camping - be outside as much as possible and limit the time everyone spends inside the trailer during the day. Pick up used camping gear to help make this comfortable - chairs, camp kitchen setups, etc. Sister can help with purchasing this stuff and haul some of it in the van. Just because she doesn't want to camp alone doesn't mean she shouldn't have to have to her own gear.
Now, you're looking at a tow vehicle and trailer. Starting with the trailer, go simple but efficient. If sister and niece (or sister alone) sleeps in van, and kids sleep in tent(s), then you just have to have sleeping for 4 (you, wife, mom and dad). Get a dedicated bed (preferably a walk-around) - when it's just you and wife, sleep there. When mom and dad come, let them have the bed and you and wife convert the dinette or use the sofa bed (whichever is more comfortable).
For tow vehicle, look for a club cab truck (full back seat). That will fit you, wife, mom and dad. The truck tows the trailer with necessary gear for the four of you. Sister hauls the rest of the family and gear in the van.
Find the trailer that will meet your needs, then start looking at trucks capable of towing it. It may take a while to find the right combo, but it can be done.
