My sister would be devastated

Some context to the story my sister is a lesbian & found her true love through her now fiancé. She lost her virginity & everything to her & cant imagine life without her. Her fiancé always considers me her sister but recently she messaged me telling me how she has feelings for both my sister & me & that she loves me. Idk what to do I haven’t even answered the message yet. Should I talk to our mom about it? Obviously it’s not ok she’s with my sister & I’m straight anyways. I’m just in an awkward position & don’t know how to go about it.