Sorry but it is you who is not getting it OP. My point is that legally, you don't have to pay support towards your SD. Not a penny. As your OH isn't able to pay his support any longer, the responsibility falls 100% on his ex to make up the difference somehow. If she can't, it's not your or your OH's problem.
Yet if it was her losing her job, your OH, or you wouldn't be expected to pay more. If she has a partner though, he would be expected to pick up the difference. She wouldn't be able to claim anything to support her children if he earned enough even if he has maintenance to pay towards children from his previous relationship.
You don't have to pay, so you don't have to worry about it, but I personally find it disgraceful that you're the one working overtime to make up his loss, meaning that all of it goes to your family rather than him working some hours when some of this would then have to go to his first child.
When my ex stopped paying maintenance, it put me under massive stress. The money didn't go on luxuries, it went on essentials. Like your OH, it started with him being sacked. I didn't see it coming, I suddenly had £300 less a month (two children) and with high childcare and a mortgage to pay, I had sleepless night worrying how I would pay both. In the end, I had to count every penny and use my credit card until I was able to go for a higher paid job. When that happened, I decided to never rely on maintenance any longer because I couldn't do through this any longer, and so my ex never paid a penny after that and I didn't pursue him.
So however much I respect how his loss of a job has affected you, I sympathise with the ex, especially as in your case, at least you don't have to pay childcare any longer, whereas she might not have an obvious bill that she gets to cut.
As said, let's hope that he wins his case and if he does, let's hope he pays some maintenance back out of it and more importantly, let's hope for everyone that he finds another decently paid job very quickly.