Blue Bloods Season 10 Episode 19 revolves around possible changes to the Reagan family.

Sean comes to see Frank with some news: he did a DNA test for a school project and found out that he has a cousin other than Nicky, so someone had a baby nobody knows about. Frank is surprised but skeptical; however, the mystery is soon solved. A woman Frank encouraged not to quit the police academy 20 years ago comes to see him. Her son Joe is a cop and she wants him to be transferred to a less dangerous assignment. She also tells him that his father was Joe Reagan.

Frank arranges to meet with Joe but doesn't manage to tell him that they're related. Later, he tells the family and they're skeptical at first but Sean says the DNA test is accurate. Sean suggests they invite Joe to Sunday dinner. Frank thinks that might be awkward but Henry seconds Sean's idea.

Joe's mother is angry when she learns Frank will not be transferring Joe to another unit and says don't contact them til he's changed his mind. However, Joe shows up at Sunday dinner anyway.

Meanwhile, Eddie gets attached to a baby she finds abandoned in front of the precinct, and Danny and Baez try to prove that a man Erin is trying to prosecute murdered her key witness.

To find out what happens, watch Blue Bloods Season 10 Episode 19 online.