Hmm.. You can rest on father's or brother's arms and feel happy.. Not like it showed her making out with him or something... So it won't be surprising that he's family.
I mean, which couple would want to bring their little brothers out on a romantic date?
Still I won't rule out that he might be her boyfriend...
As for Big sis being Ryo's mom, highly improbable.. Because of their age proximity. Unless Ryo is from the future and and they just dumped him to let her younger self take care of him.
But the main point is - Ryo knows pocketwatch stalker.