A guardianship of a minor must use the correct forms, and the order must be signed by a judge. What you are talking about sounds like a power of attorney, which may be an option that another attorney here could address. If you go the guardianship route, it is my understanding that you are not required to give up your parental rights just to consent to a guardian, so be careful about the forms that you sign.
An order and temporary letters of guardianship come from your local probate court and you can only obtain those by filing the proper petition for temporary guardianship with the probate court. While your informal agreement regarding the care of your child may be fine between your sister and you, it may not satisfy the child's school, daycare provider, a medical facility if something happened while you were away, etc. I recommend you speak with an attorney. Also, depending on your custody rights to your child and the child's other natural (legal or legitimated) parent's rights (if living) you may not have the right to override the other parent as caregiver and appoint your sister when you enlist.