Lyra's celebration of Trixie's birthday five days before the actual date - Christmas Day - so that she'll get more than one present a year.
Book II
Sunset reassuring Diamond Tiara.
A Mother's Duty
Twilight Velvet assuring Celestia that no matter what, she'd raised Sunset properly and Sunset still loves her.
Likewise, Luna doing the same.
Book III
The realization that Discord has been financially supporting Fluttershy, Angel, and Posey the whole time, even though he's the one being hurt most throughout all of this; it shows how much he loves his family, even though he's not there. Also serves as a massive Tear Jerker for Fluttershy, since she's basically treated him like crap since her parents "broke up".
Night Light and Velvet officially adopting Sunset into their family, making her their daughter and Twilight's sister.
Book V
Faust's rewrite of the timeline in order to try to give everyone happier lives.
Shimmer, who had very much thought that she would hate Sunset for "taking over her life" comes to love her as the sister she never had, to the point that she openly declares the alicorn as such.
