Can't your sister in law bring her kids and stay at your house while you are gone? Or if the kids have to go to school, etc.. can the dogs go over to her house? If she can't take all the dogs, perhaps board the other two and let her take Chester home with her.

I think Chester accompanying you on your trip may be out of the question if there are chances he will get so stressed out, etc... But I also think that the dogs getting checked on once a day is not a great idea. And leaving them to go in and out of the house as they want is a terrible idea IMO. What if something gets into your yard? What if the weather goes bad and they are stuck out there if something went wrong with the pet door. There is just too much that could go wrong with that idea.

If your sister in law is unable to help you except for coming and checking on the dogs once a day, you still have time to find someone else. The trip isn't until the end of February. Contact your vet, see if they have a list of pet sitters. Ask the pet sitter to come and visit Chester a few times a week or you take him there to get to know her/him better. Probably by the end of Feb, the petsitter and the dogs would have gotten to know each other better.

Just a few suggestions...