Frozen Tears (Part 1)

Rini ran to the open letter on the floor as she read the name. It was for her, but what was it doing in her parents' room, crumpled into a little ball? There was no time to spare before she decided to straighten it out and read what she had received. Ironically enough, it was from her mother...but not Serena.
Once again, questions surged through her head as she read on in the letter. Her mother had an incurable disease? But how was it possible that Serena was not her mother? She read the signature at the bottom, but the ink seemed a little smeared.
She had to find out the truth. Rini ran to Serena in tears, holding the crumpled piece of paper in her other hand.
"Mother, is it true that I'm not your child?" she wept.
"Honey, how'd you get that idea? Of course you're my child." Serena sounded convincing, but slightly shaky, " So you read the letter I see, it's just a practical joke, I thought it was nonsense, so I didn't show it to you..." her voice trailed off again.
Rini nodded sniffling, trying to persuade herself also, that it was only a joke. But who would do such a thing. But what if it were real? Who was her mother, and why did she leave her...
Rini put all her thoughts aside and decided to go to bed. In the middle of the night, she heard whispering from downstairs. It was her mother and father," I think she suspects us Darien, oh, what can I do. She'll never forgive us..." as the voices faded.
Rini rolled out of bed and thought for a moment, who suspects what, and why would she not forgive them...unless, it was her they were talking about. And her real mother!! She crept downstairs to see her parents sneaking out at night and carrying a box with them.
When she opened the back door, she heard a voice so similar to...dear Trista (Setsuna). She sounded sick, but when did she become so gravely ill? Why did she not mention anything to her?
And did she know who Rini's real mother was?