When Suitengu was explaining to Kamiya how he had already found his sister and she had been with him all this time (in the form of a charred bone fragment left from her ashes), it's shown that Suitengu found out that Yui has been sold as a prostitute and had become "broken" after years of abuse.
When she saw Suitengu, she started begging him for money and started to suck on his fingers. Suitengu wept a tear of blood which she swallowed before he hugged her. It's obvious that Suitengu used his power to put Yui to rest while they were hugging.
As Yui died, she was smiling. I am wondering, was she smiling because she realized the man hugging her was her older bother, or was she happy that she was freed from her life as a prostitute?