In their verdict, the judges ruled that Sónia Lima "entered the sea with her daughters with the aim of taking their lives." Lima "acted freely, deliberately and consciously [and] knew that it was conduct punishable in law", they added.
The judges decided on a 22-year prison sentence for each death, making for the maximum sentence allowed in Portugal, of 25 years.
Lima was also sentenced to pay civil compensation of more than €103,000, of which €60,000 is to go to the girls' father, "for the suffering caused".
The accused was "not yet in a desperate situation", the presiding judge ruled, noting that she had "family support". Throughout the trial, she said, Lima "never showed that she had internalised what happened."
At the first hearing in the trial, Lima said she was not guilty as charged, saying that she had taken the girls to urinate on the rocks on the beach. However, the judges took the view that this "makes no sense" given that it was a cold night on which the tide was in and the waves high.
They also cited the note found in the accused's car that said "father, sorry, I can't take any more". That was, the court ruled, "a clear farewell message".
After the verdict, a lawyer for the girls' father said that the sentence "was fair".
On 15 February 2016 Lima, accompanied by her two daughters, left her parents' home in Amadora, where they had spent the weekend, and after driving around Lisbon, stopped at the beach in Oeiras and entered the water with the girls.
The body of the 19-month-old was found on the same day, while that of her sister was found only a week later on a different beach in the area.