Should I stay with my husband or should I leave him?

My husband and I were childhood sweethearts. We are nineteen years old and recently got married. We moved in with my brother Blake and his wife Allison to help take care of our mother who had fell down and injured herself. My brother works the third shift at the mill. My husband and I stay with my mom at night while Allison stays with her in the daytime. My husband and I both do our best to take care of my mom. One night my husband said he wasn't felling well and he would just stay at my brother's house. I went to my Mom's house without him. We were only apart for one night, but that one night our whole lives would be changed forever. The next day my sister in law Allison came to my Mom's house and told me my husband Eddie made pionate love to her. I didn't believe a word that came out of Allison's mouth. I had hoped my husband would never betray me so deeply, but when I told him what Allison said he admited to making love to her. My husband say's him and Allison ate dinner together and drank a few beers. He blames Allison and says she seduced him. He claims he can only remember bits and pieces about that night with Allison because he was so drunk. He also says he even said my name when they were in the middle of it, but Allison chose to ignore that. As if my husbands betrayel wasn't bad enough a few weeks later Allison went to the doctor and told my husband she was pregnant with his child. When my husband and I told my brother what his wife had done he left her and moved into a small apartment across town. I knew Allison would use this baby to her advantage. She would use this baby as a weapon to hurt me with. She would tell my husband to rub her belly and talk to their little bundle of joy. My husband rubs her belly and she puts her hand over his hand than she says to him the three of us are going to be one big happy family and you are going to be one hell of a y daddy. Allison glances at me and laughs. She smiles seductively at my husband. My husband looks extremely uncomfortable and backs away from Allison espiecally when he sees the hurt look in my eyes. The next month I find out I'm pregnant too! The doctor heard two heartbeats that means we will be having twins! I'm very happy and excited about my husband and I starting our own family together. My husband has repeatedly begged me to forgive him for the one night he slept with Allison. He still says Allison seduced him and took advantage of how lonely he was and how much he was missing me that night. He say's he is not in love with Allison. He is in love with me and wants to raise our children together and have a long and happy life together. I still love him very much to and he is working very hard to earn back my trust. He says he will even go to marriage counseling with me if I thought it would help our marriage. I know Allison will always be a part of my husband's life because of the child they share together and he might possibly feel torn between our family and the child him and Allison share together. Allison does not make things easy for me either. She tries to turn my husband against me in the hopes that he will run stright into her open arms. Should I stay with my husband and focus on our family or leave him and make a new life for myself and the twins because I feel like my husband is not even truely all mine anymore?