My husband and I live in the same house and have no children, nor any complex arrangements tying up our finances. We each want to leave all our property to the other, and then when we're both gone the remainder will be split between my disabled sister and our favorite charity. Wouldn't it be easiest for my husband and I to write one joint will instead of two separate ones?
What you've described is the classic joint will. Two people make a will together, each leaving everything to the other. Then the will dictates what happens to the property when the second person dies. It's legal and it's got its pluses -- but most people find that these are outweighed by the minuses.
On the plus side, a joint will is designed to prevent the surviving person from changing her mind regarding what should happen to the couple's property after the first person dies. For example, if you were worried that your husband would remarry after your death and rewrite his will to leave everything to his new stepchild for ice skating lessons, the joint will idea might look good.
On the minus side, joint wills can tie up property for years, pending the second death. During those years, life circumstances might change so much that a new will would seem appropriate -- but the deceased person will not be around to approve it. The survivor will be stuck with the will as it's written, or have to go through legal hassles to prove that it fits a narrow exception. For example, let's say your husband dies first. Twenty years later, your sister has married a millionaire and doesn't need your money, and you no longer believe in the goals of your once-favorite charity. Tough luck -- all of the property that came to you through the joint will have to be passed on as the will described. Sometimes it's better just to have faith in the living half of the couple to make the decisions.
For tips on other ways to make a simple or basic will, see The Simple Will: No Frills, No Fuss, No Anxiety.