The Rio L is a bit heavier, and your sister and her kids/husband @ 75 kg. do not need the extra
I'm 77 kg. and I sail the Rio M all day long when teaching with 4.5/5.0 m2 Retro Rippers.
I use the Rio M as a "tow board" to give my students a fast track on the water beginner experience.
I've sailed the Rio S, and it's OK, but for bare beginners I think the Rio M will prove to be better.
It makes a quite good giant shortboard with the CB up so your sister's family can use it even when they get to the planig stage. Planes pretty early with a 7.5 m2 rig.
Start them out (esp. the 50 kg. kid) on something around 3.3-4.0 m2 and they will progress at a phenomenal rate.