It's odd, compared to each sibling, in Hoshido, Ryoma's quote is the only one that is quite personal to his siblings and Kamui. I thought that he was going to be really strict and serious, but he seems very sincere. Sakura's quote is about peace again, Hinoka is rallying trust and support, and Takumi is just being a d*ck.
Hovever, in Nohr, the quotes are less personal apart from Camilla's, which shows love for her brother (if not in a dark, Tharja kind of way), Marx is concerned about you and Leon's quote is more like a dig at his brothers. Elise's quote is the most interesting though, as it might refer to how in the Nohr path you have to revolutionise the entire Nohr kingdom from the inside due to troubles (which I presume is caused by Garon, the father Elise might be referring to).