After reading about so many of your situations, I almost feel bad asking my question but I'm looking for practical advice and I'm hoping that those of you who have been through something similar can assist.

My DH and I were married in 2013 after dating for 3 years.  When we met, his son, 18, lives with him mostly full-time and his daughter, 16, lived with him part-time and at  her mother's (ugly divorce in 2002) part-time.  My daughter, 16, lived with me full-time (amicable divorce in 1996).  We knew from the start we had differing parenting expectations.  As examples, my daughter did her own laundry by age 10, was expected to help out around the house, and was very independent.  My DH was still doing their laundry for them when we met, they had no chores, and a few times, when he and I were on long-planned and hard to schedule dates, he allowed them to interrupt our plans so he could chauffer them around for last minute social activities.  These differences caused a lot of anxiety and stress when we first moved in together.  

We have always agreed that while the "kids" were in university, they would be able to live with us rent-free and, as where we live is very, very expensive (rent for small 1 bedroom exceeds $1200 / month) and we have both the space and the means to assist, they could stay with us for a time after they graduate from university to save money to buy their own place as long as they were working full-time, paying minimal rent and generally behaving like reasonable adult roommates.    

In 2014, my stepson  (age 22) dropped out of school, worked a minimum wage job that had him working odd hours after which he'd play video games for hours, and end up sleeping most of the day.  He started binge drinking and leaving his beer cans and other mess all around the house.  There were too many other things to list here as well.  My husband and I agreed (after a lot of uncomfortable conversations) that this was unacceptable behaviour and put an adult child living at home agreement in place.  Things got better for a while but then things started to deteriorate.  When we put pressure on him to live up to his side of the agreement, he walked out with his laptop and the clothes on his back and moved in with his mother.  He refused to speak with his dad for almost two years and my husband was devastated. He blames himself, and I think me as well, for losing the relationship with his son.

My SS reached out to his dad in 2016 to reconnect and they began to rebuild their relationship. My husband was so thrilled to have him back in his life, he has been totally afraid to do anything that might upset the relationship.  For the first year after they reconnected, my husband kept that relationship separate and I had almost no interaction.  In 2017, SS moved out of his mother's place and got his own apartment with our financial support.   After he moved out of his mother's (who hates me), he started to come over for dinner occassionally and have a casual acquaintenance type relationship with me as well. He had a good job and things seemed to be going well overall. 

in 2018, SS entered a serious adult relationship with a not-yet-divorced woman who was 11 years older with 3 teenage kids.   Unbeknownst to us, SS also went through phases of depression for which he would not seek assistance.  About 2 months ago, these bouts of depression caused him to miss work (without calling in sick) so much that they fired him.   At the time, his girlfriend was also financially struggling as with no official separation agreement, she was not receiving child support.  Girlfriend suggested that he move in with her - which he did.  He accepted a new job that only pays $16 / hour and now is expected to contribute to the household finances - which for some reason, he didn't anticipate.  They're now fighting over money. He has blown all of his savings since moving in with her as they make poor financial choices together. His girlfriend wants him to sign a long term lease on a bigger, more expensive apartment which will allow pets.  He isn't ready to do that so has asked if he can move in as he recognizes that refusing to sign the lease will basically put an end to the relationship.   

My husband did discuss it with me before saying yes.  We agreed that this would be a short-term solution (6 months with an option to renew for another 6 months) and that we would put another behaviour contract in place.    However, while I'm worried about his behaviour, I'm also worried about my husband's for a couple of reasons.

1.  He keeps talking about how we can fix things for his son.  Last night, he said that we'd set our expecations and then hold him accountable.  However, I don't want to have to hold him accountable.  I'm launching my own business and hold two other jobs -  I want my hands on parenting days to be over.

2. His guilt over the rift means he won't hold SS accountable.  He so worried that his son will end the relationship again that I don't believe my husband will be capable of putting his foot down.

3.  When SS is around, my husband treats his daughter like a second class citizen.  It's the only time he comes across as really sexist - and it makes me furious.  As an example, a couple of weeks ago, we took them both to visit their grandparents (a flight away) probably for the last time.  My husband invited his son to go get a beer while his daughter was expected to work in the kitchen (twice!!!).  His son binge drank with his cousins and left his sister to clean up the room they were staying in while he slept off his hangover - and my husband didn't even blink that at the behaviour.   My SD and I talked about it and she's so resigned to this just being how it is when SS is around, it makes me furious.  And my husband denies its an issue.   I'm worried that my SD, who suffers from anxiety and is in the final year of university, will be negatively impacted by the change in circumstance.  

So....I'm looking for ideas about how to hold my husband accountable for the next