Revocation of a guardianship requires a judicial decree
Draft a Petition for Revocation of Guardianship.
You may obtain the general form from your county courthouse depending on what county is involved. The requirement varies quite a bit from jurisdiction to jurisdiction, but generally is a county form which requires you to state the reason why you wish the guardianship to be terminated.
Attach your medical certificates to the petition to show you are "on your feet". When you file the petition and pay the filing fees, you will be given a date to appear.
In order to terminate the guardianship, you will need to petition the court and demonstrate how your circumstances have changed so that you are now able to resume the care and custody of your son. Ideally, the termination of the guardianship would be done with the agreement and cooperation of your sister.
You should consult with an attorney who is experienced in contested guardianships.
If a court was involved in the giving of guardianship to your sister, then only the court can restore guardianship to you upon filing of a petition asking for that remedy and stating why it should be granted.
If you awarded your sister short-term guardianship of your son informally, undoing it can be informal as well, but can get ugly if your sister fights you on it, therefore landing you in court anyways.
This answer is provided for informational purposes only and does not create an attorney-client relationship. Follow-up with a licensed attorney who regularly practices in this field is strongly recommended.