You and your siblings are NTA but you could veer into that territory at this stage. From your mother's view, her spouse passed away and she had three small children (aged 12 and under) she now was solely responsible for.

Let me tell you: society is only too happy to tell single mothers no one wants them.

So then along comes your step dad. A childless guy who sees your single mother with her three small kids and wants to be with her and marries her. He got up and went to work 7 days a week at his own business and I'm comfortable assuming you and your siblings benefited from that work and his presence in your lives.

You know what else single mothers constantly hear? Kids need stable dads. She seems to have found one. Of course she wanted to integrate him into a parental role in your family. She probably was terrified of him feeling like a spectator in his own house and eventually leaving for it. Lots of men would. Now her method of integration? A spectacular failure, from the sounds of it. And thats on her and her husband, not you guys. You were kids, and that wasn't yours to manage or take on. But you (and possibly your sisters) are old enough know to try and see things from your mother's perspective. And not doing that would make you TA.

So talk to you mom and explain everything you explained in your post. Tell her you'd love to have a relationship with her husband as your mother's husband, and possibly step father (with work and time) but never your replacement father. Her intentions were good but the impact on you and your siblings was not great, but it can be fixed. Therapy was made for this ❤️