Well, "grandmother" is the accurate term here. She's the biological mother of Tia's biological mother. The term "Step grandfather" has been played fast and loose here by the media - he's the younger boyfriend of the grandmother. Although frankly, he looks 60 years old instead of 37.
But sadly, this IS Tia's grandmother. And no matter how you picture a grandmother, and what behaviors you usually see from a biological grandmother, murdering a 12 year old grand child is EXTREMELY rare even if they aren't grey and kindly and baking cookies. So yes, I agree.