Hi! I just read the original post, and wanted to comment on this subject as an adult adoptee. My bdad's rights were terminated by the courts, and bmom relinquished me and my baby sister, 10m my junior to Children's Services. I was 14m old, and was adopted just before turning 2.
My perspective on this is to be as open and honest with both the children, but absolutely with M when he asks. Saying he knows he's adopted doesn't take away the uncertainty of "knowing" your adopted. His young mind can not grasp what has happened, but as he matures, he'll have questions and he'll want answers...and mine started LONG before I was 15. His scars will run deep, and image him reunited with his bmom at 15 or 18 whichever...and she tells him she tried to reach out to him years ago...and there is a possibility that this could be construed by M as you keeping his bmom from him...and if she plays that...he could really be angry and resent that fact.
My abrother wife adopted his daughter when bmom relinquished R, she was 2. When my neice started asking questions at 10 about bmom....my abrother's wife simply told her "your birth mom didn't want you". Choosing the correct words is CRITITCAL in self esteem, and when neice finds bmom and recounts "my amom said you didn't want me". I know what happened, and when R learns the truth this will tear R up..and her anger will backlask...to her amom (and dad too) for not telling her the truth, and for defining her in such a manner.
My advice! Be open and honest as early as possible, and don't hide anything.
Best Wishes! I respect the life that you are providing for these kids, and even more..your willingness to go the extra mile in doing what's best for your kids.