Hello and thank you for your question.
 
Not all Autistic children are the same.  They vary with preferences and learning styles.  Some can transition from one activity to another with minimal accommodations while others are more "stuck" in their routine.  Despite being siblings they may not have the same kinds of learning styles.
 
First, I would have a conference with the childrens' parents before having either child return and see what are the preferred activities for each student. You may wish to conduct a simple questionnaire.  I would include the following questions. I also have added suggestions after each question...
 
1- Do they have a transition object? ( Such as something they carry with them to ease going from one activity to another). If so, then ask for the object to be in class.  
 
2- How often do they need a break?  Many autistic children can only stay with a task for brief periods of time (Unless it is highly engaging to them, such as watching the wheels on a toy car).  Also, an assistant assigned to each boy would be best to  help out.
 
3-  How long can they stay with one activity before becoming upset? Perhaps they need to ease in to their setting by both boys staying a minimal time frame and then increase their time in your setting as they become more adjusted.
 
4- Do loud noises upset them? (Including noises that are not obvious to most children such as chairs moving on a floor.) You may need to find a way to minimize noise such as cutting tennis balls and putting them on the bottoms of each chair so they are quieter.
 
5- Are daily life skills problematic such as toileting, eating, putting on shoes, etc.? Again, an assistant can be so helpful and not take away from the needs of your other children.  Don't forget that teens during the summer can be of great help as well as college students majoring in special education.
 
6- What kind of communication system or systems do they use.  Are they using only voice.  Perhaps they use verbal (speech) with sign language. Do they need photos or pictures to help them to communicate or transition from one activity to another. (Highly recommended for preschoolers anyway who are at a more concrete stage of learning)  Try and duplicate whatever language system is used in the home.  If they use sign language, try and learn some signs.  If they have a picture exchange system, ask to have it in the classroom.
 
7- What should you do if they become upset?  Ask the parent what works for them at home.  Sometimes they need to not be thrown in to a group but be a passive learner from outside the crowd.
 
8- Do they engage at all with other children their age APPROPRIATELY or are they only content to be with objects?
 
9- Are there issues with textures.  Do they only eat one kind of food such as soft and finger food (such as string cheese).  Thus, they could become upset if offered a cookie or cracker.  Perhaps you will need to get foods from home.
 
I commend your offer to help these boys in their Sunday School environment. There is no guarantee that these recommendations will help you, however, it could be a start.  Good luck!
 
There are many good websites to learn about autism and help to work with autistic children.  I have added websites below to help you.
 
 
Louise Sattler, NCSP
Owner of Signing Families
http://www.SigningFamilies.com