This isn’t going to change until you make it change. I wish there was an easier answer, but you must not allow your mother’s inability to effectively manage her life ruin what you have built up for yourself and your children. You do not owe your mother anything. You didn’t ask to be brought into the world and you have long since balanced the scales since you were a teenager. At 26 you cannot be in orbit around your mother’s life. Your obligation is to your children and they are not going to get a whole mom if you are carrying this resentment.
Pick a date for your mom and stepdad to leave and explain that this is something that you are committed to. Her anger is only designed to make you feel guilty. Don’t take the bait. Explain that you can’t have her stay and you cannot continue paying. Your mom needs to grow up and move out. Stop paying for her cellphone. Let her be angry—she might need to feel that until she can finally take responsibility for herself.
I am not suggesting you do this tomorrow. But I am suggesting that you explain what your timeline is and do not let up on it. The stress you are under will not go away on its own. You have to stay well for your children and yourself. Your mother’s anger is nothing more than attempt for her to avoid her responsibility. She might profit from Debtors Anonymous – a 12-step group for people in chronic debt. Take care of yourself now, and let your mom have the dignity of having to manage her own problems.