His ex wife can live in the house as long as she wishes but eventually it will go to our DSS
How do you know that? As I said "unless there is some solid legal paperwork drawn up", if there was appropriate legal protection drawn up, then fair enough, but then he was more or less "lending" his exw the house, not paying the mortgage for her, as he (or DSS) maintained ownership of the actual property.
And in the current economic climate, nothing is guaranteed.
Personally, I would not have wanted to be so beholden to my ex, nor do I think for children of separated / divorced parents, that contact should take place in FMH.
I can see why you think your exh has done a good thing, its not a path I would have chosen, especially if it meant I didn't have enough money to take maternity leave with my children, I would feel they were the ones who had been deprived of my time.
I just have a different view of things, I do think though, that given the financial situation you are describing for your DH, it is unfair to expect the OPs DH to be able to contribute in the same way, not to expect him to - it may have been the right way for your DH, but it wouldnt be for everyone, and I especially do not see why the DH here should be paying the mortgage on a house there are other children living in and a new husband, from what I read, the exw, the new husband, the older DD and older DS are not his responsibility - so 4 out of the 5 people living there are not his responsibility.
You may have been happy with the situation, but I wouldn't have been, I expected to get of my backside and go to work when exh and I split up (in fact I waited until I had a promotion and a FT job before I left to ensure that was the case).
It smacks of financial control, and to a certain degree preventing the ex from moving on with her life, all under the guise of being a martyr or possibly guilt, guilt (even unplaced unnecessary guilt) makes people do strange things.
We all make life plans, I certainly never got married with the intention of getting divorced, but life changes.
If you ask me, you have had no choice but to be happy with the set up as is, if you want to be with your husband.