The way you initially address your parents-in-law can have a lasting effect on them and can shape the future of your relationships. Every family is unique, so here are some basic guidelines of etiquette to keep you in safe territory until you figure out what works best in your own extended family.
If you can bring yourself to call your parents-in-law Mom and Dad, they'll probably be pleased. In many families, parents consider sons-in-law and daughters-in-law to be as close to them as their own children, and they appreciate that affectionate regard in return. But some people find this practice difficult, at least at first.
The safest tactic is to confess your uncertainty and ask your parents-in-law how they wish to be addressed. Be prepared to honor their response. If they ask you to use their first names, do so. If your mother-in-law asks to be called Mother Smith, so be it. If the answer is Mom, call her Mom. When everyone's parents are present, you may call your own parents Mom and Dad and your spouse's parents Mother Jones and Father Jones.
In all cases, using a pronoun instead of an actual name is an absolute no-no. When the person is within earshot, using words such as she and her is definitely not courteous, and the more you use them, the more rude they seem.
You can usually address your spouse's grandparents with their last names appended, as in "Grandma and Grandpa Smith" (unless there is no ambiguity, in which case you can call them simply "Grandma and Grandpa"). Some grandparents don't wish to "sound so old" to their adult grandchildren, though. Ask directly what the grandparents prefer to be called.