Some posters here have commented about the potential for future trouble with your intended direction. By constructing this second residence, occupied by others, you are migrating from a property you solely own to some level of joint ownership / shared ownership. You will lose some significant rights to your land and property when that takes place.
You might consider negotiating a buyout clause or some sort of provision that would govern if things explode in the future. It's hard to envision, when things are going well, and there is family harmony, that a bitter dispute could erupt. But circumstances do change over time, and they will change-- you just don't always know how. The differential in ages could trigger differing points of view about how certain things should be handled.
I was previously involved in a family joint ownership that worked well for years. Then it slowly degraded over time, finally resulting in years of expensive battles in court. Early in that process I had the opportunity to create such a safety net-- but was convinced I would never need it. After all, we are family .... right?