For what it's worth, my advice would be for you to get all the interested parties together. Just explain the situation to everyone, ie, the trouble, the hassle, the cost, etc. Tell them that any monies that you have to spend on their behalf will have to come out of the overall inheritance, the same with any bills or anything else. Also say to them that when it's all over and done, you should all meet up for a nice meal, family gathering. If it ends up that someone of the family doesn't pay their share, or whatever, it's no big deal really. The amount you'll be disadvantaged for will be minimal, just do it and if you don't want to do it, ask for some other family member to complete it. The thing is, one of them will.
As a matter of interest, what do you consider reasonable in terms of your time, etc?