Check the meaning of the terms that you use? A "yearling" has been alive for at least a year - in case of a deer, has got through its first winter. Those fawns that you see with the doe are probably this past spring's birth - so not yet yearlings, not yet got through a winter. Her last spring's fawns, which she would have "booted out" when this year's fawns came along, would now be "yearlings", if they are still alive.
I do not know if does will "adopt" another fawn over winter? We see here in Western Manitoba / Eastern Saskatchewan about March that the deer seem to congregate in much larger herds than the rest of the year - probably have used up last summer's fat, into very hard times until some Spring new growth. A couple bad blizzards about then will really knock down the numbers. A period of melting that softens the snow cover, then freezes, allows coyotes to move very easy on that frozen crust, where as sharp hooved animals like deer break through and bog down. If that doe has two fawns, she has been eating fine for a period of time - that is a good sign that feed is available that she likes. I think fawn survival includes some amount about adult leadership, but also food supply, weather, predators and so on. Big herds make spread of disease pretty easy - I know Western Sask had very large effort to control CWD (Wasting Desease) that did not really control it - pretty much let that prion spread out across the west.
