Sylar hums and slips his fingers up into his hair, enjoying the attention even if it's a pretext for having a silent conversation.

Or you could end up killing him if you don't get it right this time. He has faith in Mohinder's abilities, but he also doesn't want to see Peter dead. Hurt, maybe, for being the constant thorn in his side, for kidnapping Mohinder and breaking into his memories, but not dead. There's no point to it now. Peter is prey without adequate natural defenses -- a lion reduced to a rabbit. But you could tell if we're related or not based on a blood sample?

For all he knows about people and the way the human body works, he has no idea what a person can or can't tell from blood save for a few things that might be artistic liberty from the occasional rerun of criminal dramas on TV.