Name one situation where someone wouldnt be disposable
The fact is...there arent any. Every one is disposable....no one is indispensable in any situation
People go to extreme measures for the ones they love. People will often make sacrifices they wouldn't ordinarily make or do things they wouldn't ordinarily do to preserve those they love. I suppose one could also develop a high enough level of empathy to treat others as family as well, and be willing to make the same sacrifices for them. An example would be brothers in arms.
Now, your proposal is a bit more primal than that. When it comes down to it, if you can separate your mind far enough from the relationship, then I suppose one could sacrifice the well-being of anyone, including family, in the name of self-preservation. One of our biggest weaknesses as a species is the ability to care, love, sympathize, empathize, whatever you want to call it. Think about all the superhero movies. The bad guys always get a leg up by attacking the heart of the good guys. Attacking the mind can be much more effective than a physical attack.
So yes, on a primal level, there is no situation in which no one is disposable. It just depends if you have anyone that you would choose to jump in front of a bullet for and be willing to make the ultimate sacrifice.