I don't have a lot of REALLY close friends in real life. My best friend for about the past year has been the best friend I have ever had and probably will ever have! We just clicked right from the start of becoming friends. We have learned so much about each other and I truly see him as a brother. We are so close people question our sexuality when we are together.
When we first met each other (but before we became friends), I always told him he should date my sister if he and his girlfriend ever broke up. Well within the past week he broke up with his girlfriend of almost two years and my sister broke up with her boyfriend of almost a year. Since both of their breakups, both parties have expressed their interest in the other to me. My best friend wants my permission and is really cautious of the issue due to the fact that he is literally shaking when he talks to me about it and he cried one night after we talked a little bit. My sister has talked to me about it as well but isn't as cautious because she knows no matter what she is my sister and we will be good, however she was going to reject him because she didn't want it to affect my relationship with him.
I have given him my "pre consent" (he wanted to ask me before he officially asked me), and I have told them both that I am in support of it. However within this past week that they have been flirting over text I can already see a difference between me and him! He texts her more than me and his conversations are actually filled with substantial content rather than just the occasional checking in with each other. I don't know what to do anymore! I don't want to say anything and ruin what they could possibly have but I also don't want to lose my two closest friends! This is probably the worst I have felt in a long time!
Even if your friend begins to date your sister, I doubt you'll "lose your friend". Of course things could eventually get awkward if they were to break up, but if your friend truly values your friendship he wouldn't let anything that could potentially happen with your sister ruin the friendship the two of you guys have.
I kinda think this is why he asked you for permission in the first place LOL. You need to ask yourself if he's treating YOU differently than he would if he was dating someone else. Of course he's texting his girlfriend the most, but if he still treats you the same, as a best friend, then there's not really an issue idt.
Everything is new and fresh and exciting to your sister and best friend. Give them time to get settled in together in whatever happens. Stand back pray take deep breath and all things will work out for all 3 of you if it's met to be.