Complex issue in my experience as different crew seems to require different approaches. The Admiral gets uncomfortable over 15 deg and starts leaving teethmarks in the stanchions at 20 deg. She's new to sailing, so heeling is not second nature. Letting her take the helm is marginally effective, as it lets her have some control of the situation. But, as
@kappykaplan indicates not heeling beyond her comfort zone is the safest solution. Works ok for me since my boat doesn't really go faster on her side.
The neighbor, a long time friend, was concerned that the boat was going over (as is the Admiral). Also, new to sailing, this guy is no couch pansy. Motorcyclist, mountain biker, could do a standing back flip when we first met (30+yrs ago). I talked him through ballast, self-righting, etc. and then told him that the boat would round up before it went over. Even though he understood the theory, it still took a gradual approach to get him comfortable near 15 deg.
His wife was another story. Absolutely no apparent concern for heeling. She was at the helm, while pointing (as much as my boat will point), and she made an adjustment in the heading that turned into more of a reach. Of course the sheets were cleated and the adjustment resulted in a cabin rearrangement. I looked at her, and she was still grinning. My neighbor asked her why she wasn't more concerned. She said, "it's Jim's boat; and he's not concerned, so I figure we're ok".
My cautious daughter was a little concerned at first, but her first trip was 20+knots, gusting well over that, and a small craft advisory. We were heavily reefed, but still heeled over 20 on many occasions. She had to learn and help, and I told her that I needed her help (which was somewhat truthful). She handled it well. I think that knowing she had to rise to the occasion helped her push her fear back. She has been out on better days since, and seems to be comfortable. Maybe a trial by fire can cure the apprehension, although it may be a less than humane approach.
The other daughters don't seem to have a problem with heeling, short of being dumped off the boat...
In summary;
Let them take the helm, or control the sheets. Show them how adjustments effect heel.
Per
@kappykaplan, don't dip the rail.
Project calmness. Don't yell, or get excited-even if they dip the rail and re-arrange the cabin. This should be fun for all.
Keep them thinking about something; course, speed, trim, making lunch, etc. so they are less focused on heel.
Gradually ease them into increasing heel angles.