Need help with the relationship

Patient: Hi, My name is Shubham. I need your help as i am feeling stuckand really not able to know what to do further. As far as my life and problems are concerned, it is as follows. I was 11 year old when i lost my father. My Mom has brought me and my sibling (elder sister and an elder brother). we were financially ver poor at that time. My relatives at the starting helped us but then they started showing the attitude. My mom had suffered alot and faced so many problems for us. When i turned to 14, i came to know the bitter truth about my sister. My sister who was 19 at that time, was having a affair with my cousion’s husband. It was really difficult for me to accept. i thought alot but could nt able to say it to mamma as i dnt want her to suffer more. Me and my mom are very close to each other. She loves me the most. That person helped us financially but me and my mom were neveer able to accept him as a person and because of that we started having tensions at home. One day me and my mom discussed about it (When i was 22) and we decided that we will not accept him. There used to be so many problems between my sister and my Mamma. i feel really bad that i could not able to save my mom from the abd words taht my sister used to say in anger. My sisier is very short tempored and after that incident i could not able to have a friendly relation with her as i always keep o thinking that she is lying. 2 yeras before we came to know that my mom is suffering from cancer and i lost her last year which has shattered me completely. The clashes between my Mom and my sister din’t stop and it continued till we lose her. But my sister is the one who stays with my Mom at home as me and my borther are working. That person helped us very much at that time.i am full of guilt that i was not able to do anything for my Mom and could nt give her a healthy and happy life. Today, the problem is my sister wants me and my brother to get married and she wants us to settle down to Australia with her. But me and my brother are not able to accept this realtionship and not able to get what should we do. We know she is our sister and she wants good for us but her relationship with that person is untolerable for us. We tried to tell her but she never listens to aynything and says what we are doing is right. i dnt knw i feel like if i tried to accept this relationship then might be i will hurt my Mom very much. I have no confidence left in me. I am completely broken. i want my sister to live happily. But we dont knw how we (me and my borther) will be able to adjust with both of them in one house. I am relly not able to understand whether i should just break all my relation with my sister or i should just accept it and leave it all on destiny. Please help. i really dnt wanna my mother upset with me. I mis her alot and could nt accept till now that she is no more. i keep on crying as of helplessnes..i hope i am able to understand you my problem.Please help. please