If I understand the information we have available everyone the parents listed as possible suspects have been questioned and theoretically cleared (for now, obviously). We know this kid had DNA drawn, but this doesn't mean anything in terms of guilt or even suspicion of guilt -- the crime scene guys would want his DNA to help them identify samples. They likely secured DNA from everyone who has visited the home and was willing to provide a sample. Right now they are looking for DNA that they cannot identify.
Consider what we (think we) know. Do you really believe this kid walked in the unlocked front door of the home (how did he know it was unlocked?), turned on the lights (because why would he want to be sneaky right?), grabbed a few cellphones and maybe make a call on one (probably updating his twitter account: "lulz breaking into the neighbors house now lulz!"), maybe paused to grab some milk (the jerk probably drank it straight from the bottle!), then for no particular reason he decides to kidnap and murder a kid (if the teen did it the infant is dead, right?). Does this scenario make even the slightest bit of sense?
To me it does not. To me it sounds ridiculous. Your mileage may vary.