Issues with parents as an adult

This may be a little long but I really need some advice in this situation it's been causing so much more stress for me in an already stressful situation. I'll give a little back round so hopefully it's easier to fallow along. I'm 30 and my mom ans step dad( who raised me since I was 2 months old so more my dad then my bio dad ) have been separated for 15 years almost ( still legally married) both had addiction issues in the past his family blames my mom for it pretty much.. Anyway now into where it is causing a problem... my step-dad is currently in the ICU on a ventilator after a bowel perforation this week and now is sepsis. His sister contacted me to let me know what was going on so I could come and see him because we aren't sure what the outcome is going to be. So my mom demands to come with my siblings and I which I was hesitant about but I knew we were going to the only ones at the hospital at that time so I let it slide.. she has the nerve to try an pull the wife card.. got mad at me and my sister for being the ones to talk to the dr instead of letting her in the room also. Now mind you they haven't spoke or seen eachother in 15 years she had him removed from our home by the sheriff with a restraining order when they split.. he ended up on the streets an eventually back with his mom an sister's. Anyways his sisters have made it very clear to me that my mom is not welcome around like when his mom passed his sisters made sure I knew my mom was to not be ay the funeral. So now he's been in ICU for 5 days now and everyday she asks when we are going back up to see him.. idk what to do shes putting me in the middle of a situation I don't want to be in, he's my dad to me we are extremely close and this is very hard on me, I feel like even when I tell her I don't want to get his sisters mad at me for bringing you there or having you two run into eachother there his family is going to be mad at me. And then it is jeopardizing me being able to go see him because if he gets better he will be going back home to recover at his sisters house. I understand my mom just wants to make sure he's okay an thats fine I don't mind that but I don't want to bring him more stress either by bringing her around him. Idk how to explaine to my mom in a way she will understand an not just think I'm picking his sisters over her. Please give me some feedback.