You need to take one thing at a time. Attempting to stop them from attending the meetings with their mother will be counterproductive.
But you have every right to study with your own kids. Try this.
To get everybody used to the idea, let them know you will be studying with them from now on but you will sit in on their study with the MS for a month or so to give them time to adjust to the change and for you to observe how he is doing it. Then after that have the MS still attend but you conduct the study and inform the MS that he is free to stay or go after that.
Invite the the M S to go fishing or hiking with you and the boys and you might even wake him up aswell.
Patience and diplomacy.