What was the motive given at trial?
I've read the siblings were having an incestuous relationship and JC caught them in a compromising position? Is there any truth in this?
It can't be certain what sparked the violence but João Cipriano (Leonor's brother) had a history of violence and criminal conduct, he had only recently been released on parole in respect of an earlier attempted murder. It can only be surmised what drink and drug fuelled event took place. Whether Joana returned and found them in a compromising position is merely a theory, only Leonor and João know what actually occurred and they won't say.
What little evidence there is appears to point to the child being knocked about, probably by her uncle, it doesn't take much to kill a child if her head is hit off a door frame. Leonor took the blame initially and claimed that her brother's only involvement was in hiding his niece's body. She later changed this to save her own skin and putting the blame on her brother.