You are doing great. I had the same issue with my son. I even think he was waking up about 10 - 15 times. My advise... just hang in there. Before you know it, he will just start to stay sleep longer and the visits to your bed will become less and less. It seems as if this will never end, trust me it will. It just takes some time.
As for you taking care of your niece / nephew, try to get some additional family help with them. If not, during the week for school, maybe the weekend, someone can give you a break.
Again you are doing a great job, just think of the difference you making in these children's lives. You are a Blessing to them.
Good Luck
-R.
Your are a Blessing