I dunno about the reward part, but I would suggest a family meeting to figure out rules and expectations together. If they are part of creating the "rules" they will have a much higher degree of buy in. We do that with ds and he does it at his school. At ds preschool all the kids (and staff) work together to establish rules and norms and those kids are 2-6 years old!
I think trying to set it up where everyone decides what works together, you might not need to reward them for doing it, they may feel more ownership over the rules/norms/rhythms and not need the extrinsic motivation.
Hey! I'm a Rebekah, too! (and a fan of Portlandia) Cool!
I like the idea of a Marble jar as a way of keeping track (you could put a bird on it!), but I guess I wonder how long that will last? I do this sort of thing for my students (if they all speak in English all class long they get a tick. If they get 15 ticks in a term we get a pizza party) but I have them for one year and it is within a term they have to reach their goal. If I had to do that indefinitely, for years and years I would get bored, and I think they would too. Every time I have tried sticker charts or the like for DS, he always just forgot about the chart and wound up just adopting the habit and I'd end up taking him somewhere fun just because he was doing so good. I would also worry that if they had competeing jars, rather than a cooperative jar it might breed some animosity there between siblings, and cause in-fighting...but I'm just thinking to my childhood here...My brother would have pummeled me (or threatened to pummel me) into not doing my chores, just so that I didn't have more marbels that him, or he would have dumped my marbels down the gutter just to teach me a lesson. But, hey, if you think it'll work, go for it!
I think depending on your kids, obviously, that by age 5 they can take their dishes to the sink. Keep their rooms relatively tidy with some help. Lay out their clothes and possibly dress themselves for school. Make their own snacks and clean up after themselves. Take care of any pets. fold laundry. Brush teeth, wash face and body, bathe. Be expected to treat each other with kindness, and all circumstances being fair,ask for things in a quiet and calm voice without whining (though I would be loathe to reward for that as it might teach a child to repress unhappiness to get what they want which is sort of counter intuitive to me) ...Each could have one important task to help care for the house that they do on weekends, like weeding the garden, or doing the dishes (depends on how clumsy your kid is!) or vacuuming the rugs...That's possible at age five, but they'll likely need reminding.
Is that what you had in mind, or something bigger?