did ben kenobi already know luke's family was dead when he saw leai's message?

Beru and Owen were normal "humans" so Ben wouldn't have detected that they had died unless he was constantly monitoring them with the force, so no he wouldn't have known they were dead when he saw the message, he might have become suspicious when they found the Jawas that the Droids had been bought from dead by what clearly was Storm Trooper activities, and likely wasn't surprised when he was informed of them being killed. The only thing that could have tipped him off was if Vader had went to Tatooine, but the fact that he didn't since him in that close of proximity, which he would have, is evidence that Vader hadn't been involved in the actual killing only in the orders to interrogate and kill for the information on the droids.

---It's possible he sensed something wasn't right, but wasn't positive. It's kind of like when the starship destroyed that planet, and he felt a "disturbance" in the force.

no but probably suspected it at the sand crawler