K.F.
Write a will so that guardianship is a done deal. If it is a relative and you have a clear will it would take a great deal for anyone to prove she is unfit, etc.
I don't know what the criteria is for you and/or your husband becoming incapacitated. I would do some googling ad then consult an attorney to determine how to set up guardianship in the event that something other than death made it impossible for you to care for your child.
You can save yourself money by drawing up the papers yourself and having an attorney look them over before they are signed and notarized.
Also, while your sister might be a good mom, do consider whether she would be a good steward of the money. Sometimes you want to separate these duties if you know someone that will be responsible and see that the child always has what is needed.