Hi Amy
Why don't you just send a letter addressed to "The current occupier" at her address, enclosing a stamped addressed envelope? You can explain you are trying to find out if your relative is alright. Hopefully she will reply herself, or if the house has changed hands the occupiers might know what has happened to her. If you have just sent cards, unless you put your address on the back of the envelope they couldn't be returned to you.
Jan