My best friend dating my sister
You state in your letter how you care for them both.
Perhaps all you need to say to them for now is this and that you’re glad they are happy.
Writing about how you feel could be a useful outlet, remembering these very strong feelings may subside as you get used to the situation.
If you’ve a friend or family member who you can trust to share your concerns in confidence this may be useful, although you’ll want to avoid this escalating into a family drama.
Now, before you think I'm crazy and not making an ounce of sense, Listen to what I have to say.