40-year-old Stepmother refuses to pay 20-year-old stepdaughter's college rent despite paying for her own 19-year-old daughter's rent and being married to the stepdaughter's father for 12-years: 'My husband has no idea I'm paying my daughter's rent'

Blended families are tricky, and there's no set rulebook for when a stepfamily relationship should or shouldn't or will or won't be the same as a biological one. It depends on the people, their pasts, their stage in life, their compatible personalities, and their willingness to accept new family members. Still, when you've agreed to a marriage with another person, you've agreed to split everything, including finances and the care of your now shared children. It wouldn't really make sense to make a secret arrangement without one child without telling your spouse unless that child had money from another parent or other side of their family, exclusive from your own blended family's situation. 

Generally, the stepmother, and original poster of this topic is in the wrong here, simply for making a commitment to a large ongoing expense without consulting their partner. The very fact that her husband was kept in the dark about her agreement with her daughter puts her squarely in the wrong here. Besides, she's been the other girls stepmother for 12 years, since she was 8 years old and for the majority of her life, that's more than enough time that she should well and truly consider her stepdaugher to be her own and the woman and her husband together should be giving bother the olderst girls the same arrangement. 

Sure, if she wants to go through the same process with the stepdaughter as she did with her own daughter in order to get the cost down and teach the stepdaughter the same lessons about concessions and budgeting, then go right ahead but by keeping her husband and stepdaughter in the dark, she has willfully created the issue here. What she frustratedly refers to as a "whole 'thing'" that she resentfully blames her husband and stepdaughter for is actually entirely her own fault.