I try to avoid these types of backgrounds where I can. If anything, sometimes I put them in there as "stand-ins" until I can come up with something better, at which point I rewrite it. Some of the background stories I've come up with are a little "out there" I admit.
For example, my character Allison comes from a village that was abandoned by humans for reasons unknown for a long time. Many many years later, what appeared to be a deadly "disease" epidemic struck the animals living in and around the remains of the village. Some of them thought it was a curse, some blamed the new travelers coming through. You know what it was? A severe allergic reaction caused by the pollen of a flower that exists only in that area and only reproduces every several hundred years. Yeah. A flower.
Is it a sad back story? Well I suppose from some perspectives, she lost her brother but she doesn't go around moping about it. Like most people who don't know their parents, siblings, grandparents who die when they are really young don't really give it much thought or really care, and she doesn't either. Not everyone died anyway. She's moved on and its an interesting perspective to have a "traveler from another land" that had a sort of culture all of its own whereas before, she probably would have never got the chance [or in this case excuse as everyone wanted to leave the village] to leave and live out her dreams.She'd still be stuck in the village doing every-day Joe type stuff which is not what she wanted.
Another I've made is for Fate and Spelunker who are an imaginary evolved subspecies of a maned wolf that lived inside of a sealed off mountain range. None of the canines had wings and none of them could get past the mountains surrounding them. The pass that let their ancestors through was supposedly closed and forbidden to enter. Most didn't bother to question this due to the number of earth quakes and rock slides around there so it wasn't safe to go up there. The problem for Fate was [who is the older of the two siblings] that as the amount of resources began to decline one year, by her leader's order they were being forced to cut back on their food intake, she was finding it harder and harder to feed Spelunker, often shorting herself of food just to ensure he got enough.
Going against her leader's order, she went up to investigate the mountain pass hoping they might find a way through to a better place where they could find food/water/shelter. When she and Spelunker went through it, part of the cave collapsed, separating the two siblings, Spelunker being the one closest to the inside of the mountains while Fate was on the other side. Neither one of them knew if Fate was trapped but ordered Spelunker to go back before it collapsed again and continued to wander the cave where she did find a way out. Is that tragic? Well I guess to some it might but that's only looking at the glass half empty. I don't consider it very tragic at all because Fate did find what she needed/was looking for after she got out of the tunnel and was happy about it even if it means there is some small part of it that kind of sucks and can't be changed.
So while the parents are missing/dead in all three of these character's backgrounds, is it a relevant piece of information? Not really, and its hardly the first detail I would want to include. One revolves around the blindness and power of nature of in action and the other two's backgrounds revolves around questioning authority and finding a means to provide for one's self and managing to find it. You don't /have/ to explain what happened to mom and dad or the entire pack.What happened to the rest of Fate's pack? I don't know. Maybe they did die, maybe they struck gold and are thriving now. Maybe they found a way out too, who knows? And quite honestly /who cares?/
Both good and bad things happen, not usually just one or the other. The background figures/characters don't necessarily make your character who they are and you don't have to come up for an excuse as to why and how they vanished from the picture. You would probably be a lot better off you didn't. If you're going for a semi-problematic background, don't have your character openly mope about it or better yet, make something good come out of it to offset it. Most people's lives are not so horrible that at least one good thing did not happen to them.