I suppose you could get a reality show and make a fortune to send the kids to college at least. But this is too crazy even for reality TV.
I feel your pain. So many people don't have good relationships with their stepfamilies, and here you have a rewarding friendship with your stepsister. What would it mean to you to sacrifice that? Probably a lot. It would be a loss. So, since you get a lot of value there, you should keep it. Now, you can decide to discuss everything with her except your stepmother/her mother, but that would put a barrier in the relationship. Maybe it's a beneficial barrier - only you can decide that.
You can also make a pact with your stepsister that neither one of you will share your family confidences with her mother and your father. That means, if Dad and Stepmom come to dinner, you do your best as you did this last time, but if doggie bites daughter, you either don't tell stepsister that part of it, or she swears not to tell her mother that she knows nothing about the evening.
Or, you hang up with Stepmom about 30 minutes sooner than you did this time, saying you aren't getting in the middle of their relationship, that you've answered the issue and it's done. Yes, Stepmom can say that either you or your stepsister is lying, but she ignores the 3rd possibility which is that she herself is lying. But my sense is that your father and stepmother are not going to change, they are the way they are, and somehow or other they are getting a payoff from it. She does, on some level, know she's wrong, because she told you not to tell your stepsister she called. So you have to walk that tightrope (in part because you don't know if she called your stepsister back and said, "2kidsmama said you're a big fat liar"), or you have to decide that you're talking to whomever you want, and that people have to own what they say and do.
I'd talk it out with the stepsister and decide if you both want to invite the ongoing wrath of stepmom/mom, or if you are going to have a pact to safeguard each other's shared confidences.
I have a wacky family with a nasty and emotionally abusive side (and some physical stuff), and sometimes I get down on myself for not being a better person or more "together". My husband says he's astounded that I'm as normal and strong as I am, given what I came from. So I think the same could be said for you and your stepsister!