OP, I have no advice about step-parenting as I am not one. I can understand that 'parasitic' teenagers are about as bad as it comes.
Look at it another way. You have brought this wonderful spouse into your life. Then comes along two teenagers who have probably not 'chosen' this for themselves. You did not automatically want this but this is part of your wonderful spouse's family. So you have a two dynamics going on within your household - see I get that this was somehow your investment that contains this duo-dynamic household. Your spouse, and your spouse plus two offspring.
The strength you have here is your original relationship with your DH. A good tip I was once taught when dealing with 'spineless' people we have in our lives including spouses or say, irrational/shitty/spineless colleagues or bosses, is to recognise their weak spots and support them regardless. Pick up the slack, let them know they are supported, treat them with kindliness rather than complaint.
In short, support your partner.
In terms of the teenagers. Accept them and allow your dh to cope with them. Support him in whichever way, they will not be there forever.
I have not read your previous threads, it is possible that I am repeating pp.
I'd say keep going, and that you CAN transform this situation.