It’s so easy for everyone to pressure you to take a dog you don’t want when it’s not them that will end up taking care of it.
You have already had the upheaval and extra responsibility of a child and it’s not fair to expect you to take a dog and be the villain if you say no. The only person that deserves to be made to feel bad is your step sons mother.
I feel deeply sorry for you OP having your life turned upside down like this, because of the circumstances you will appear the bad guy if you complain but your life is going to be a lot harder now and it’s inevitable that you are going to end up with a lot of caring duties for a teen and a dog unless you put your foot down.
If you do decide not to take on the dog you are going to be resented and blamed for making your step son more unhappy, it won’t be his mother that people focus on for making him unhappy it’ll be you.
I’d be having a long hard think about if you are prepared to stick with this long term, look at the step parenting board for an insight into your future.
I agree with a pp who noticed it’s all about boundaries on this site until you actually decide to set one, people reply without imagining themselves in the same situation.
If your husband is planning on taking full responsibility for parenting his son and looking after the dog without constantly relying on you then I’d maybe consider it, I’d make sure it’s not just words though and he actually means it. Dogs are a lot of work and change your house completely- especially big ones.
They get hair everywhere, they smell, they restrict you for holidays and even days out and having guests, they cost money, need walking in all weathers, they bark and can upset neighbours and unless very well trained they jump up on furniture and can be destructive and get hair all over your clothes. They aren’t a low maintenance pet.
If I was in your shoes now I’d be worried this is the start of a pattern where everything revolves around your step son and making him happy to make up for his past trauma and your needs will cease to exist.
I am fully sympathetic to your DSS after having a traumatic childhood myself and I know this will sound harsh to a lot of people and like I have no heart at all.
I just noticed that the majority of responses didn’t take your feelings into account at all and made you out to be heartless for not wanting a dog and extra responsibilities. I think this a sign of what’s to come for you and I wanted to balance out the replies.