At the risk of being hugely pedantic, if the estate is left as to the residue equally between your hubby, b-i-l and charity X, you don't own a share of the property at all. You have a "chose in action" to have the estate administered. IE this means, until the estate has been administered the only rights you have to enforce are to have it properly administered!!! (Yeah, makes perfect sense to me too)
What your hubby should do is take legal advice with a view to seeking directions of the court under CPR Part 64. The executors have a duty not to bring their personal views and their duties to the estate into conflict. The estate cannot be administered without the sale of the property, thus all sensible offers should be considered (in line with the executors duties to maximise the estate).
Best bit of advice is dzug - get one to buy the other's share of the estate out. Would save a whole load of hassle - although you can't force this.