My [37] half sister [34F] says my husband [38] tried to cheat on me with her

Details about my relationships with my Husband and half sister:
I?ve always been protective over my half sister, Leah, as both our dad and her mother passed away when she was a teen (her mother when she was 14, our dad when she was 17 and I was 24). My mom has always loved and been kind to Leah and even took her in for a few years after the death of Leah?s mom. Leah hasn?t always been so appreciative of my mom mainly due to missing her own mother and believing that my mother stole my father away from hers (to clarify, my parents split when I was 3 and my dad briefly dated her mother for about 3 months and got her mother pregnant before they broke up, soon after the break up my dad got back together with my mom and they got married 4 years later but Leah?s mother never got over my dad) but Leah and my mom have a better relationship now after Leah has seen that her mother wasn?t always the best person. Leah is a fairly dramatic person which means a lot of people struggle to get along with her however her heart is in the right place and she has always got along with my husband Josh until now and was always friendly and inclusive towards him (as he is very shy and reserved).
I met josh through mutual friends while at uni (he was studying to become a hematologist while I was studying mathematics). We first met at a get together with friends where he didn?t really speak to me much so I figured that he must be very shy, however after the get together said friends told me that he had a crush on me and wanted my number, he is extremely good looking and when he did speak he was absolutely hilarious. Soon after this we began dating and dated for around 2 years before we got married (we have now been married for two years). I Thought that he was the most amazing person that I had ever met, he is funny and kind, but now I think I might have been wrong.
Current situation:
Recently Leah and Josh have become quite close, she spends a lot of time with him when she comes over and at family gathering. I didn?t think anything of this until i returned from a Christmas celebration with my moms family (Leah didn?t want to attend and josh had a thing with his parents) on Wednesday to a huge number of calls and texts from Leah telling me to come to her place and it?s an emergency. I rushed over to hers and she told me that we needed to talk. She sat me down and told me that my Josh had tried to make an advance at her while I was gone and she didn?t know what to do she said that she went to my house to walk my dog (as she does every Monday) and he just came onto her. I just ran to the bathroom and puked my guts out. she said I could stay with her if I needed too, I just said ok but I needed to go home to get my stuff. I felt sick the whole way home and when I got home josh was there.I didn?t say anything at first and he told me that he had something to tell me, i said I already knew and he asked me what I was going to do, I told him I didn?t know, however then he said something about cutting Leah off, which confused me, I asked him what he meant and he said that Leah came onto him when she came to get the dog and assumed that I thought that as well. I don?t know who to believe, right now I am staying With my mom who is going to give me as much time as I need, but I am so lost and I just don?t know what to do. I feel so sick. I love Josh so so much but Leah is my sister and she wouldn?t lie to me.
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TL;DR My half sister says that my husband came onto her, while he swears it was the other way round.


Source.