Dashed line = Adopted relationship (sibling/parent)
Single dash = Dating or half-siblings
Msn Heartbroken Emote Broken heart = Divorced
(Trees are read as left to right ^^)
First Row:
-Yaar is married to Yaiga. They are the parents of Yama.
Second Row:
-Yabir is married to Yama. They are the parents of Yona, Yuri, and Yyla.
-Seafoam is married to Beach Breeze. They are the parents of Sandbar and Coral Cove.
Third Row:
-Yyla, Yuri, and Yona are siblings. Yona is married to Sandbar, and they are the parents of Seasmoke.
-Sandbar’s sibling is Coral Cove.
Fourth Row:
-Seasmoke, adopted son of Sandbar and Yona, currently works as a florist. Lives in Ponyville.
I think that’s just the artist’s style.