C.M.
First of all - congrats to you for doing all of this alone!!
My first question would be about your brother and sister. If your dad cannot afford to take care of them, I'm sure you could 'foster' them and then you would get money each month - that may help so you could get a bigger place - maybe one more bedroom so each sibling could have their own room and your two small children could share. Just a thought.
As far as teaching you sister about things and being a motherly role - you said she was raised wtihout a mother. Were you around her while she was growing up? I would be open and honeest with both your brother and sister about what you expect from them. They are old enough to help around the house and respect you and your children. I would not put too much on them in regards to watching / taking care of your kids.
You say you and your children's father cannot afford to live together. Are you together? If so, it would be cheaper to have 1 place for rent/utilities/groceries than two each month. If you two are not together, he should be paying you child support. I do not suggest trying to live together if you aren't together as a couple.
As far as being stressed - it's understandable. Make time for yourself each day - even if it's a bath or a 10 minute walk after the kids are in bed. For your kids - it sounds like there has been a lot of changes (aunt/uncle moving in) so I wouldn't worry so much about teaching them to respect, but I would teach them by modeling. How do you treat your brother and sister? Their father? Your father? ETC. Kids learn by seeing what others do. Reward for good behavior - make them think/feel like you WANT them to succeed and do well (instead of always pushing them for what they are doing bad).
As far as your siblings yelling at them - you need a clear and concise line in the sand as far as what the rules are. Your kids are confused as to who is the authority figure. If you don't want brother and sister yelling at them or punishing them, tell them that.
Good luck! You sound like you have a lot on your plate!