She can pay you and your wife for "care". She cannot count the room and board as you are not a licensed facility. You should draw up a contract stating the services you provide, and she needs to pay you by check each month the same as she would if outside services were coming in to the house.
What you also need to keep in mind is her qualifying financially. You need to determine what her "countable" income is after she pays all her allowable medical expenses. Essentially you want her to be as close to zero at the end of the month. Look at this posting as to how you arrive at that figure.
http://veteranaid.org/forums/index.php/topic,8.msg8.html#msg8