Well, honestly, it seems you have already kinda made up your mind.... you seem to really want the job.
Will your brother in law and you be working together, or just for the same company? If you both can keep business seperate from family, I don't see why it can't work.
As for the daycare families you have now, how much notice do you need to give them? Will the job you want be willing to wait until you fulfill your contractual obligations to your daycare families?

Good luck with what ever you decide to do.