Suggestions:

~For some reason, Snape, Lupin, Harry, and Draco have to pose as a family. Likely none of them are entirely pleased about this. (Think: We're the Millers)
~Snape and Lupin team together platonically to look after the boys in some way
~Snape, Lupin, Harry, and Draco all find themselves in a situation where they are siblings in some capacity
~Molly Weasley/another Order member/Hogwarts staff has decided that Snape has been alone quite long enough and knows the perfect person for him. (Alternatively, he/she/they have tired of seeing Lupin alone, so forth, yada)
~Harry and Draco decide to team up to unite their parent figures/mentors/guardians/favorite professors (Think: The Parent Trap)
~Harry and/or Draco finds himself in charge of two de-aged professors; the other insists on helping for whatever reason
~Any other scenario you can think of (I could go on for some while, so it's really best I stop, now...)

Note: Not everyone need be an official part of the immediate family to satisfy this challenge