Yue and Yukito Start Running Out of Power
SPOILER!
Becoming Sakura's guardian changes Yue's source of power. Sakura is still developing, and the magic she can supply is not enough to sustain Yue and Yukito indefinitely. Yukito begins eating more, sleeping more, and visibly weakening while Yue admits the problem to Kero. The danger is easy to miss because it first looks like exhaustion rather than a magical collapse.
How you can tell
Yukito's constant hunger stops being a running character detail and becomes a warning that the body shared with Yue is running short of energy.
Toya Has Known Sakura's Secret for a Long Time
SPOILER!
Sakura spends much of the series trying to hide Kero, the cards, and her nightly adventures from Toya. Toya is far less unaware than she thinks. He can sense supernatural presences, recognizes Mirror even when the card is wearing Sakura's face, and understands that Yukito's condition is connected to something Sakura cannot yet fix. His silence is protective rather than oblivious.
Why it matters
Sakura is not carrying the magical side of the family entirely alone. Toya has been watching the cost build even when Sakura believes she has kept him outside it.
Toya Gives Up His Power to Keep Yukito Alive
SPOILER!
When Yukito finally collapses, Toya gets the private moment he has been trying to create and transfers his own supernatural power to Yue. The sacrifice stabilizes Yue and keeps Yukito from disappearing, but Toya loses the ability that allowed him to see spirits and sense the supernatural world so clearly. Sakura overhears enough to understand that her brother knew far more than she had realized.
What changed
The power shortage is solved by a family sacrifice Sakura did not ask for and cannot undo. Her growth as master has affected people far beyond the card battles themselves.