Mother/daughter relationship

My parents got divorced long time ago. I rarely saw my father. I lived with my mom and grandma. When I was 11, my mother left our small town in Europe to seek a better life in NYC. She promised she would take me and my grandma with her as soon as she could. The next time I saw her I was 17. We moved to a small apartment building with no elevator in Brooklyn. My mother told me she got married to a younger guy and was expecting a child. She told me that she told everybody including her husband that I was her sister and not her daughter. She also told me that her husband would divorce her if he found out she had kids. 10 months later my mother had a baby and them another baby boy. I and my grandma were helping her with the kids 6 days a week. We adored them. 7 years passed. Kids were calling me their aunt. On my constant question of what I should tell them, my mother told me that the truth should die with us. About 8 months ago I noticed that my mother was getting very irritated and constantly angry. She would pick up a meaningless fight and every time I would try to tell her something contrary of what she thought, she would tell me that I wouldn't see the kids. One day we had a fight and I was told that I would never see the kids. I was very hurt as well as my grandma. I decided that I shouldn't call my mother for sometime so that she would understand that she couldn't manipulate me with the kids. 6 months passed without talking. Not a single phone call to me nor my grandma who is 69 years old with heart condition. I was taking care of my grandma as well as supporting her financilly in full. I saw that my grandma was suffering without seeing the kids. She would talk about them all the time. One day she went to their school so that she could see them for at leat 5 min. I couldn't take that situation anymore. I called my mother and suggested what we didn't argue anymore. She agreed but told me that her husband was against us seeing the kids. When I tried to call him, she interfered and told me that he hated me and that he didn't want to talk to me again. When I told her that I never had an arguement with him, she told me that he hated me because I was trying to break their family. I told her that all I wanted was for my grandma and me to see the kids because we loved them. She refused to allow us to see them. She suggested that we shouldn't talk for some time. I think it's important to know that I'm 28 years old now, young professional who fully supports myself and my grandma. I was recently engaged to a very nice guy. He thinks my mother is my sister. I live in the world of lies. I can't even tell him nor my friends of what is happening with my family. I'm very scared about what is going to happen to the kids in the future. My mother can betray them as she betrayed me. Should I just leave her alone and never talk to her or should I fight for kids visitations for their sake? I'm about to start my own family. Should I tell my future husband the truth? I'm embarassed and hurt. Please help. Thank you