Twins.
It is the first thing Tyrion thinks when he sees Jon Snow and Arya Stark embracing for the first time. He knows they aren't, he knows that Jon isn't even Arya's full brother, but they look like twins. Their coal dark hair and shimmering grey eyes the complete opposite of Jaime and Cersei, but the oneness of them is the same.
Twins worry Tyrion.
He glances sidelong at the Dragon Queen; she doesn't seem to understand the danger here. She sees a brother and sister happy to see one another; she doesn't see the deep connection he can see. She believes she is now first in Jon's heart. She believes he will do anything for her.
Tyrion knows she's wrong.
When Jon introduces Arya to Daenerys he keeps his arm around his sister and does not touch his Queen. When Arya, seemingly innocently, asks her what weapons she's trained in and Daenerys replies "dragons" and Arya replies with "oh" he knows without seeing that the wolf-girl is both disappointed and disgusted with the Queen's lack of forethought.
Tyrion likes Arya.
When the welcome feast begins Jon seats Arya next to him and they whisper and laugh together. The Queen starts to notice something is different now. His other sister, Tyrion's once-wife, makes it clear that she does not like Daenerys but also that she is jealous of Jon and Arya's closeness. When Jon and Arya leave together and cannot be found until morning Tyrion knows the Dragon Queen has lost her "king."
Tyrion pities Daenerys.
Samwell reads the book that tells of Rhaegar and Lyanna's wedding to the gathered forces and everyone knows Jon is the rightful heir to the throne. He may not be a Stark but he is THEIR king.
Tyrion fears Daenerys' wrath.
The dead are defeated and Arya is the hero. They throw a feast for her, but she disappears. Jon leaves to find her. Daenerys legitimizes Gendry and betroths him to the wolf-girl. Jon finds her shooting her bow. He leaves, returning with the Maester, they wed in the godswood.
Tyrion watches history repeat.
Samwell reads the book that tells of Rhaegar and Lyanna's wedding and everyone knows Jon is the rightful heir to the throne.
The battle is won, Arya is the hero, and Daenerys is empty. She's lost her oldest friend, her "king," and her glory all while in the North. She hates the North and its people. She wants to burn them, burn them all.
Tyrion fears for the world.
Jon and his sisters betray the Dragon Queen who seems to have lost her mind. They warg into her dragons and destroy her armies. They then release the dragons and watch them fly away, it is rumored they have left for Essos. Daenerys cuts all her hair and drinks poison, the Dragon Queen is gone.
Tyrion feels he should feel worse about this
Arya disappears for a month, when she returns Cersei is dead. Jon accepts his throne because of responsibility. They name Tyrion Hand of the King.
Tyrion does much of the ruling.
Arya dies in childbirth. They are twins, with lavender eyes and coal dark hair. She names them before her death; Visenya and Brandon. Their father gives them all the love Arya couldn't.
Tyrion is old and wise, he fears for the future.
At six and ten the twins run away to Winterfell and are married in the godswood. Their father understands; the people don't.
Tyrion sees the cracks emerge in the kingdom.
Dragons return, one for each twin and they relish in their power. Soon there is talk of rebellion. Dragon fire stops it. Tyrion's pleas for mercy for the rebels end in a fiery death.
Twins always worried Tyrion.