Mr and Mrs (or Ms) So-and-so is all you need. There's no need for anything more formal than that. Attorneys don't have any special title. Remember, this is for a Christmas card, not a formal occasion.
How about Mr. and Mrs.
Just address the card to Mr and Mrs (husband's name). It's perfectly okay to address the husband and wife but to avoid offending the wife just do the Mr and Ms last name and you're probably safe.
you dont
Type your answer here... Mr. and Mrs. Mark Henderson
If not directing the card specifically to the daughter in this case, usually you would address to "The Family of [deceased]".
If it was one of the parents of the husband or wife's or child and you know the two people well you would put 'John & Jane' (the card says the rest) and sign it with 'My sincere sympathies to you and your family ____________. If the husband has passed away then address the card to the wife 'Jane' or 'Mrs. Jane Doe' and if it's the wife that died then address the card to 'John' or 'Mr. John Doe.'
addressing attorney and wife
No- that would constitute forgery. A wife cannot sign her husband's name to any legal document unless she was granted a Power of Attorney and states on the document that she is signing his name as his attorney-in-fact.
the wife
It is etiquette to put the wife's name first and inside the card put her name first and then the husbands (the deceased was his mother-in-law) so he should be included.
Just the one whose parent it was. Unless their spouse was extremely close to their inlaw.
Your estate will be responsible. Indirectly, you wife will either have to pay it or get a smaller inheritance.