I don't think it was a pre meditated murder if it at all was a murder.And they are doctors, they have the knowledge about the body etc. Its like Harold Shipman i guess.
IF (and thats a big if by the way), they did kill her, I think it would have been accidental, and they then tried to cover it up. Thats unlike Shipman. They were very, very wrong to leave a 4 year old with her siblings on her own. That is horrendous parenting to be honest.
Personally, I'm not sure what to believe, there was always a quiet and perhaps a darker side in the wife. Although, I don't think anyone can say that they are guility or not guility without seeing the evidence that the Portuguese police have.
The DNA in the back of the car does not actually have to be from her body directly by the way, it could be from a cuddly toy of hers etc.