Sister of Mercy is a Sheo-model, a generalized character of a girl who is so good that there is no need marrying her. To be more precise she is not that good. No. She is actually good neither as a mistress, nor as a wife.
She is no good not because she is bad but because that’s what she thinks of herself. She probably waits for a boy to offer her everything. Or she may be shy thinking she has no right to demand. Or she may feel that she does not deeply excite him yet (as she hasn’t learnt the first four lessons of Columbine sister, for example).