Use the obstacles to your advantage.
Dealing with calm, level headed humans is an issue - they'll be more sure of themselves and less likely to make mistakes. That's why we begin with....
Step 1: I'm so tired!
The CCTV and motion sensor combo means anything larger than a small dog will likely trigger the alarm and then be found. The cctv will likely be pointed into the room meaning blind spots directly below the cameras and behind/under furniture. So position yourself behind some furniture, away from the direct line of sight of the cctv and set off the motion detectors. Alarm goes off, everyone wakes up. There's no intruder to be seen, so they check the cctv. Nothing. Dad says maybe it was a mouse, he'll put down some traps tomorrow, everyone goes back to bed. A few hours later and the alarm is going off again. Doing it too frequently to begin with isn't that helpful, starting with once or twice a night is ideal to disrupt the sleep without making so much of a problem that they dig for the cause. Do this over a week or more, slowly increasing the number of 'false alarms'. Eventually one of 2 things will happen. The family will die of sleep deprivation (unlikely) or, they will disconnect the faulty motion sensor system (likely). Assumming the latter, that makes the next step possible.
Step 2: Mummy, mummy, I'm scared.
Everyone is tired, and on edge but the motion sensor system is off so tonight they'll finally get some sleep. As soon as the girl is asleep you can get to work, take another toy, ideally a doll and tear it a apart. Put all but one of the pieces on the floor out of vision of the CCTV. Take the final piece and get it some degree out of the girls room, towards the boys room. If there's no CCTV in the hallway between the rooms, then place it in his room, if there is, slide it along the floor towards his door. Then back to the girls bedroom. Time to wake her up, any way will do, so long as it draws her attention to the destroyed doll (make a scratching noise over near it until she wakes, pick up and throw a piece into the pile from cover, whatever works). The girl wakes up to the destroyed roll, screams her head off and wakes the extremely tired family. It doesn't take long for mum and dad to point the finger to the boy who loves to kill dolls. One of two things happens depending on the temperament of the girl: she's either convinced to go back to sleep, in which case we repeat the process tomorrow night with another doll or, she refuses to sleep so someone (mum/dad/girl) suggests she spends the night in the panic room. If its the former, we continue destroying toys and waking the girl in the middle of the night until she makes the move to the panic room. Now it's as simple as....
Step 3: I think we're alone now
Assuming the girl likes us (the murder doll) she will presumably want to keep us safe from her brother and take us with her into the panic room. Now we have ~6 hours to kill a single young girl that's locked in a cage with us. I'm assumming no cctv inside the panic room (it's usually the other way around), so we can be as brazen as we like. Choking is the obvious get out of jail free card. Wait until she's asleep, tie her hands to the bed and shove another toy down her throat. Don't worry about the fact she's tied up, you can untie her before the morning once she's dead. This takes us to....
Step 4: the family is ruined
The death of a child is horrible, but it's worse than that. When the autopsy is performed they see the rope burns on her wrists. The poor girl was clearly being kept tied up, probably in that 'panic room', more like a cage when you think of her being tied up. There's no way to know which parent was responsible, if not both, but they sure both knew it was happening. The boy isn't talking about any abuse, but he has violent tendencies that suggest an unhealthy environment. Social services take him away. It'll be a few years before he calms down and acts more rationally but eventually he becomes more within social norms. The parents get convicted of murder, the prosecution can't prove they choked the girl, but leaving her tired up unable to help herself was enough to get them thrown in a dark hole for the rest of their lives, what jury wouldn't convict. Cut to 2 years later, a package arrives at the boys adoptive parents house, a gift from his parents... they hesitate to let the boy have it, but on inspection it's just a toy doll, with a note that says it was his sister's favourite toy.... cut to black