Here are a few thoughts on a 12 day attempt across the JMT: I'm recommending you take your brother-in-laws advice.
Twenty miles / day means about 12 hours of hiking per day including bathroom breaks, pictures, rests, filling water bottles, eating, and taking in views. Here is a summary of a typical day then:
* 6:00am - get out of tent, eat, and take down camp
* 7:00am - on the trail and hiking or taking breaks for 12 hours
* 7:00pm - reach stopping point and set up camp
* 7:30pm - start cooking or eating
* 8:00pm - about 30 minutes of light to enjoy the rest of the evening
* 8:30pm - get in tent, talk and fall asleep between 9-9:30pm
Some backpackers love that type of camping. My experience as a 13yo was that I liked more time to play around a lake and take in the fun of the sierras.
One other consideration about doing more than 15 miles / day. I've found that people that do not backpack regularly (carry 35lbs for 30 miles over two days at least once a month) wear out after about 4 days of hiking. I am sure it is partially physical, but it is most apparent on the mental side when people start to not want to get up in the morning and are unable to enjoy the beauty of the outdoors.
If you, your son, and your wife are all into pushing hard on a trail -- GO FOR IT!!! and just have an alternative plan if you find that someone is not having a good time a few days in.
One more thought. If you go easy the first 6 days and then speed up the last 6 days, you'll likely cover more distance than you would if you push hard over the first 6 days. That has always been my experience. I think it has to do with both physical aspects and becoming efficient on a trail.
Best of luck to you,
This is good way of seeing all the different parts of the trail. I took pictures of the entire trail and put it into a 40 min slide show going from south to north: http://vimeo.com/7851505