Grace Livingston Hill 
Stranger Within the Gates 

Sylvia’s impulsive young brother has come home from college with astounding news—news that throws the entire Garland family into confusion: He is married, and to a most unsuitable young woman named Florimel! But Sylvia determines that she will reach out in love to the difficult and selfish young bride.
But Florimel has plans of her own—plans to get her hands on her young husband’s inheritance. And in the process of carrying out her plans, she stirs up such strife and discord it seems that the Garland family will be torn apart, a fact that leaves Sylvia overcome with despair.
And Sylvia’s despair only intensifies when the crisis at home seems to leave her no choice but to turn her back on her own chance at love…