Why do you think Lavinia is malicious about whether Isannah will leave Cilla?

Lavinia is so "stuck" on her own self-importance that she cannot stand to see Isannah love anyone more than she believes Isannah loves her.  She also has been trying to make Cilla look bad to Isannah so that Isannah will go to London with the Lyte family.  Ultimately she offers something that completes dazzles Isannah who has learned that money can at least buy comfort.  The way Lavinia has been running down Cilla, Isannah cannot possibly value her sister and the poverty she would come to if she stayed in Boston.

she doesnt want her to leave because they where so close before.