I (26f) think I ruined my relationship with my boyfriend (26m) because I thought he was cheating on me with my sister (21f).

Tl;dr Basically what says in the title. It seems I was wrong and he was planning something else with my sister.
His name is Daniel . We have been together for 4 years . He is working in another country . He usually leaves for 2-3 months and then has a 3 months vacation . He is an amazing person and I love him very much. Daniel and me talked about marriage and we both agreed we want to get marrieed.
My sister name is Alex . I have to say I have always been jealous of Alex . She is looking better than me , is funnier than me , smarter than me and got into a better college than me .
Daniel and Alex have always had a close relationship . They are both intrested in video games , astronomy and have the same tastes in music . They would sometimes go out together when I was at work and I never had a problem with it because she is my sister after all and I trust them .
Something changed 1 week ago when they started to go out almost everyday . I would see my bf texting her and smiling , he was also very antentive of his phone . You know , the usual behavior of someone that is cheating .
Yesterday I had enough when he went out with her and came home a little late . We started arguing and I saw that he got two photos on his phone from her . I told him to show me his conversation with her. He refused . At that point I was 99% he was cheating on me with her. I told him that if he doesn't show me his texts with her I am going to break up with me. He gave up and showed me .
It seems I was completely wrong . He was planning to propose to me and he was going out with my sister to pick the perfect ring for me . He was also planning a vacation for me and him where he wanted to propose me . He asked my sister for help because he wanted everything to be perfect.
I felt like an idiot and left and now I am staying with my parents . He and my sister have been calling and messaging me . I am afraid I ruined something beautiful that he wanted to do to me . What should I do?


Source.