In my opinion you will find strangers less difficult than your relatives. You will also find that young children (especially those familiar to you) are some of the most difficult.
I would start with the simplest circumstance possible. This would be an adult in a simple three quarter head and shoulder pose. Do this indoor under controlled lighting, whether natural (which I prefer) or artificial. Once you have some success in this regard then move to half figure with hands and begin to move out from this center of gravity.
If you cannot find a subject, then set up a vase with an apple or orange. Treat these objects in a serious way, compositionally, background considered, defining form. You will find that these lessons will translate directly to your purpose.
Sooner or later someone is going to put you in to this situation. I say practice it on your terms, on your turf, and as often as you can without all the pressure that you presently feel. Like anything else, success won't come looking for you, and it is vary rarely accidental.
I continue to believe that every photo study done, whether used as reference or not, is a valuable lesson in composition and design (even a simple head and shoulder pose), and composition IS King. My best advise on the mechanics of this subject is
here.