In the FMA Wikia it says this...
In the first anime series, the remains of the failed human transmutation that was Trisha takes on a larger role. After the brothers rush away from the scene looking for help, the deformed, living mass that was created manages to crawl away. Patiently waiting not far from the house was Dante, who fed it the Red Stones necessary to give it a human form.
Which leaves the impression that Dante knew that they would perform human transmutation on their deceased mother. Isn't it a bit strange that Dante is so conveniently close to the house when the incident happens? It seems to me that she knew this would happen.
So how did Dante know that the Elric brothers were going to bring back their mother Trisha? I also can't remember when this happened in the series. Please list the episode in your answer if possible.