For Mrs. Bennet, a woman's worth is determined not by what she is but by what her husband has, and it would be a great shame If any of her daughters should remain single.
Why the word "should" put in the sentence ? What's that means?
"Should" can be used to express a conditional meaning or possibility: If any of her daughters were to remain single, as CrossRoads says, it would be a shame.
3 formal (expressing the conditional mood) referring to a possible event or situation.
‘if you should change your mind, I'll be at the hotel’ ‘should anyone arrive late, admission is likely to be refused’