First off, hello, new to the subreddit!

This seems like a good place for me to look for some advice on a situation I'm in, and I'm not sure what so do about it. Little bit of background, I live with my brother, his wife, and their mutual friend (brother went to college with her, wife is childhood friends with her). We've known each other for some time now, and occasionally my brother and his wife will go out of state, usually to visit family. This leaves me and the roommate alone for some time. Typically we watch shows and movies together, invite friends over, and she offers to get/make me food. All in all, a good friend!

Recently, however, I've been having dreams and fantasies of her and I getting intimate. Typically, I've been able to shrug it off, as I'm not sure how to act on it, but the more time goes on, the more I'm having these fantasies

Let me be clear: I wouldn't want to seek a romantic relationship, as ultimately I see her as a good friend. But I have a lust for her, and I'm not sure what to do about it. She has a casual boytoy that I am friends with, they fool around, but I don't believe they're serious.

Ultimately looking for advice on the situation, should I just wank it off and move on? Or if I wanted to act on this, how would I approach her about it? Thoughts would be appreciated!

TL;DR: I want to F*** my roommate