The story he tells Ryuu when they're in the hotel, about his mothers family not accepting them and being a cult of extremists is a lie. Daniel got Ryuu when that woman threw Ryuu to him and told him to save “this child”. I don't know how much Daniel actually knows about the family. Indeed, they may actually be wealthy. But Daniel isn't his biological father like he wants Ryuu to believe and he never personally knew Ryuu's biological mother.
Generally it's safe to assume that EVERYTHING'S a lie until this manga is finished (I saw the KING OF POP, now I can withstand anything 'til the end)
http://www.mangago.me/read-manga/papa_s_assasin_ryuunosuke_wa_tonde_yuku/mf/3a3ab6ec8923cad9/Vol2_Ch9/pg-22/
I think he just doesn't want Ryuu to know he was an assassin previously. "Son...I have to tell you the truth. You are not my son. I was in the middle of a job when your mom gave you to me. I didn't know her at all. That's why these guys are after me."
Ryuu "What? Then where's my real mom and dad"
Daniel "...Probably dead."
Ryuu "What kind of job was it?"
Daniel "..."
Im a bit confused. So he really is his parent? I got the wrong idea on the beginning...