Eric Martin took to social media this week to air out his grievances about the case playing out in Puerto Rico ... starting out by saying that just because someone takes out a protective order under this statute -- which is admittedly serious -- does not necessarily mean the alleged victim is telling the truth ... and that the facts still must be vetted thoroughly.

Eric says he's on the side of justice and truth, but by his rant here -- you can tell he's defending Ricky, and casting doubt on their nephew -- whom he's ID'd as 21-year-old Dennis Yadiel Sanchez. He also says IF the allegations against Ricky are true -- which he's incredulous over -- the response from society should not be retribution ... but rehabilitation.

That seems to be more of a general thought, though, about instances of incest ... because, again, Eric clearly doesn't believe Ricky did this.

He makes one last point in another video … namely, that he too believes their nephew suffers from mental health problems -- which he addresses directly to Dennis, whom he says he loves. Eric also says Dennis is estranged from the family, and that he knows what kind of hurt he's inflicted on other family members before all this ... but Eric doesn't elaborate.

As we reported, Ricky has called BS over all of this ... insisting his name will be cleared in court. He reportedly faces upwards of 50 years behind bars if convicted of the charges.