If you realize you aren't able to fulfill the calling 100% then you shouldn't accept it.
If you really don't want to do the calling then you should let the Bishop know. If the Bishop understands where you are coming from, he might feel its not a good calling for you, or he might tell you to try it out.
Its good to be open and honest with the bishop about the calling. Bishop always wants input.
Ok well I guess I would have to say ask heavenly father first he's the one giving you this calling the bishop is only the message giver and unless you are not in good standing with Heavenly Father by reason of church standing I don't think you should not take a calling. (Remember the choice is yours to do what you want!) Every calling I have had I Have never understood what I was doing (Mainly because I didn't grow up in the church) I only knew one thing there is a reason for callings and that is Heavenly Father gives us (Helps) to help our (Spirit grow ) and our faith grow and our knowledge increase :). WE are only given what God knows that we can handle no more than that and I don't think he would give us any less. I am stating this from what I have seen sence I have been a member what I contribute in life makes life grow and think. If I do nothing for anyone I am no good for the celestial kingdom. And just think of all the possible callings you will get by accepting one calling
For me, honesty is the best policy. The Bishop is supposed to ask about your family situation FIRST, so then you can share your concerns, if he does not then you can still share whatever circumstance may stop you from accepting that calling. I don't think is a "sin" to reject a calling when you really have a situation.
Robvmtr, I guess the topic we could discuss is: Are callings the only tool available to serve and help others? Can they be a positive tool but can they also be a distraction from doing the most important things we should be doing to feed the hungry, etc?