I can't stop lusting after my girlfriend's sister

I've been with my girl for around a decade but since her sister turned 18 a few years ago, I'm finding myself more and more attracted to her.

I'm not interested in a relationship with her, cause she can't control her feeling & her current relationship is a nightmare mainly because of her. I'd just like to see her naked & fuck her a few times. I doubt I'd act on anything asshe can't keep any secrets.

I've stored some risqué pics of her from Facebook on my computer. She got changed at our house for a night out once & left a bag of clothes to pick up the next day. I waited till everone went to bed & crept down to the bag. I found a bra & panties that smelled fantastic. I jerked off to her pics while smelling her panties, Ive never cum so hard in my life.

Since then I've wanted more. I brought a hidden camera and set it up in the bathroom cause she changes alot at our house for nights out. Her whole body wasn't as great as I'd hoped but I've seen her smooth bald pussy & her ass, yet to see her tits though.

I've also jizzed in her tea a few months ago, I was worried she'd taste in but she drank it all down.

What can I do next?????????? She keeps a spare pair of panties in her bag, I might take them if I get a chance & can get away with it.

Does anyone want updates??????