It's ambiguous grammatically.
However, your listener either knows Melanie and knows she is not your sister, or your listener doesn't know Melanie, in which case I would assume it's your sister. I would make this assumption because if the listener doesn't know Melanie, then Melanie needs an introduction as well.
My college roommate Melanie, my sister, and I ....
In real life, we interact and get clarification when it's important.
You can avoid ambiguity in written language with a tiny rewrite.
My sister Melanie and I live together.
I live with my sister and Melanie.
Melanie and my sister are both my roommates.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.