Am I wrong for this?

My sisters husband went off to date another woman. He still wants to watch and care for the kids. But she won’t let him and depends on me! I get it. I understand the grieving she needs to process the break up and needs help. I’m there for her 110%. But I’m going through things stressful as well with bills due to this COVID shit messing everything up for me and my immediate family. I have a 10 month old son and a fiancé and our own home. I go to her house 7 days a week for the last few months to babysit and I don’t ask for ANYTHING. I’m on the verge of a breakdown and close to one as I’m writing this, today. I try to vent to her and she always cuts me off or talks about how her life is much more shittier than mine (I have depression and terrible anxiety. I just want a day to cry and cuddle up to my hubby and babyboy and just cry and get everything out of my system. I tried asking her to get her ex to watch them and she starts this whole fit and attitude and argument with me telling me “ I told you, you can’t just stop watching my kids, I needed you and your letting your little emotions stop me from being able to go to work tonight that’s fucked up”. Like what? You’re supposed to be my sister!! I’d understand if her ex wanted nothing to do with her or the kids but he does!!!!! Like what the fuck. Am I in the wrong please someone just giving me advice cause I’m hurting 😢☹️( I know this is In The wrong group idk if there’s a family group I can post this to)