I hadn't realised that your mother has a 7 year old ds and IMO you shouldn't underestimate the potential seriousness of her fantasies about 'Adam'.
However, it's probable that she will be in denial that her fixation is the cause of any problems in her personal or professional relationships or that it impairs her perception of reality.
It may be that, if her GP is not supportive or your dm is unwilling to attend an appointment with you in tow, you may be best advised to call her local authority's social services department and speak to an Adult Services social worker.
You'll most probably be told that there is nothing they can do unless the adult (ie your dm) approaches them but, at the very least, you can ask them to record your concerns.
If you form the opinion that your db is being adversely affected by your dm's fantasies, I would suggest you contact the local authority's Childrens' Services in order to safeguard him.
Should you need to take such action, your dm and your stepfather will undoubtedly need reassurance that you are acting solely from concern for her mental and emotional wellbeing and that you have no other axe to grind.
I have a feeling that you will not find it easy to win your dm's confidence in this matter, particulary as it seems that others in the extended family do not share your concern.
If it proves politic to beat a retreat after a couple of days, at least you will be able to console yourself with the knowledge that you tried to be of assistance.