In addition to cooking and cleaning, contribute as much financially as you can towards other bills like power, phone, cable, etc. But even more than that, do your best to adapt to their habits so that your presence isn't throwing off their daily routine.
Figure out when they shower and plan yours around these times so they can continue to shower at their normal times.
Make it clear whether or not you'll be around for meals - especially dinner - so they aren't waiting for you or cooking for you if you have no plans to be there at the time they normally eat.
Don't invite people over; make plans to meet them elsewhere.
Clean up after your son - make sure he doesn't leave toys around. Make sure he's quiet when they are sleeping and that he doesn't disturb them in general. Of course he can play and be part of the family, but he shouldn't be disruptive.
During the times that you're all home together, have a good mixture of spending time with them and staying out of their way. You don't want to be around constantly invading their family time and privacy, making them feel like the have to always include you. But, you don't want to spend all your time in your room either, making them feel awkward or worried that you're feeling too much like you're a burden to them. So spend some evenings hanging out with them and other evenings staying quietly in your room.
Also, don't spend any extra money! Your sister will get VERY bitter if she sees you coming home with new clothes, spending a lot of money eating out, or just generally spending money that you don't have. If you need her support right now and can't afford to live on your own, then you need to watch every single penny and not spend when you don't need to. This doesn't mean you can't ever go out to dinner or a movie, but you really need to limit it. I know I would be really irritated if someone was staying with me long-term to get back on their feet, but then was spending their rent money on things they didn't need.
Finally, give your sister (and anyone else who lives there) the opportunity to keep an open dialogue with you. Tell them up front that you really want to be a good house guest and to let you know if anything you're doing is frustrating or annoying them, or messing up their routines.
I hope you're able to get back on your feet quickly.