Do they know ?

I have a question my oldest and my middle are from the same father but I never told them that I kept it from his dad and him cause before he was born he got another pregnant and he asked me if the baby was his I told him no now hes three and when my oldest comes he goes to him right away does he know there blood my mom told me blood will always know so does he know or has a feeling to love him more ? And they don't look like each other at all cause my oldest doesn't come for months and when he does he wants to hold him and hug him a lot and not my other ones