Single entry Schengen visa, destination added

• It is common for Schengen visa to grant both extra days and some flexibility in the dates, so that you won't need a new visa just because a meeting is re-scheduled by a few days. Check that the extension fits on both criteria, both the changed entry/exit date and the changed total days.
  Be careful -- you have a first date of entry, a last date of exit, and a maximum number of full or partial days. You might not be allowed to use both the earliest possible entry and the latest possible exit on the same trip.
• You may adjust your itinerary with a Schengen visa after the visa is granted, as long as the main premise remains the same. If you do have your conference as planned, then adding a family visit does not change the main premise.
• You must not misrepresent your itinerary to get your visa, or to change which consulate is responsible for issuing it ('visa shopping').

So in theory, this is possible. In practice, you should take care that you do not appear to have misrepresented your itinerary, even if you did tell the truth.

• It is good that you arrive as planned, where planned, and go to the conference first. I expect you will have documentation on that.
• Both the new and the old itinerary would have been handled by France. So there can be no question of 'visa shopping.'
• It will be good if you have your departure booked, even if it isn't exactly as originally planned. Bring enough money for the extra days, even if you will stay with your sister, to avoid questions in that regard.
• It would be helpful if this isn't your first Schengen visit.

There should be no problem unless there is something about your application I don't know.