And don't forget weight and balance!
As a PPL student, you'll learn about it and have to calculate it on the skills test, but it's not usually uppermost in your mind during training because it's just you and the instructor (or just you) and you just asume that you are within limits. After all, the instructor wouldn't go up if there was a problem, would he?
Doing w & b calculations for real is vital if there is any chance that you will be at or near the limits. Get it wrong and you will regret it!
Since you say "relatives" in the plural, I'd suggest you train on a 4 seater and try to hire the same type once you have your PPL. Otherwise, if you learn on a 2 seater (e.g. Cessna 152) it might take a few extra hours to familiarise yourself with a 4 seater and satisfy the owners that you are competent to fly it.
And since most 4 seat light singles can't be flown with 4 adults plus luggage plus full fuel (and even 3 can be pushing it, especially if your airfield is hot and high) you need to consider how much you and your relatives weigh and how much luggage they and you want to carry.
Planning roughly what this weight would give you in terms of maximum permissible fuel (and whether you would be within balance limits) for the aircraft type you want to hire might be good to know up front. But it could only be rough and ready it this stage. Plus, if you haven't yet started on the course, you'll need help doing the calculations, unless some kind soul has put them on a spreadsheet.
I seem to recall that there was one floating about for PA 28s a couple of years back.