I think they both need to be served but fortunately, you know where they both live, so I doubt service will be an issue.At this point, even if tenant says "I promise both of us will move out by June", the mother has already expressed her intent to be difficult, so I would take anything the mother and daughter say from here on out with a grain of salt. 

I got an email inquiring and, unfortunately, this person is one of few who have expressed interest since we posted the house 2 weeks ago.

I've expressed interest to the owners that I'd like to buy there property from them when they are ready to sell.

Paragraph 14B(3) of the California Residential Purchase Agreement says "...However, if any report, disclosure or information for which Seller is responsible is not Delivered within the time specified in paragraph 14A, then Buyer has 5(or__) Days After Delivery of any such items, or the time specified in paragraph 14B(1), whichever is later, to Deliver to Seller a removal of the applicable contingency or cancellation of this Agreement.If you cancelled within this time frame there's nothing more to talk about, the contract is cancelled. 

And whenever the Code requires you to give notice to someone, always opt for CMRRR (certified mail) and hand delivery if you can.

In most states you still need to send them a accounting statement for the deposit to their last known good address get delivery confirmation from the post office, even if they don't get anything back.

If you are going to mail check with the post office on what is the LIGHTEST LOAD mail delivery days of the week?

@Jonathan BowenIn Pennsylvania, the relevant statutes have an express exception for such situations.

Or perhaps a nicely written letter expressing my sincere sympathies would be ok?

You can read more about our fight in the San Antonio Express newspaper searching John Vogt in Boerne, Tx.