At times, Mariana seems as if she's simply been corrupted by César and is only following his lead. The flashbacks tell a different story, as we see Mariana learn that Sara is carrying César's child. The pair then go on a walk where Mariana contemplates pushing Sara off a cliff before ultimately conspiring with Elroy to get rid of the girl. It is Mariana who suggests Sara try parasailing and Mariana who hands the knife to Elroy in the hopes he'll tamper with the harness.
We don't know if Elroy actually used the knife but we do know that Mariana cleaned up the scene of the crime and pretended to call an ambulance for Sara. It's also hinted that Mariana murdered Elroy as he lay in the hospital so that the truth would never be revealed. She certainly has a strong motive for being the killer, seeing as Sara was messing around with her husband and dating her son, but could she also be innocent? She's not the only Lazcano with a reason for wanting Sara dead.