I'm obsessed with older women
Is this a secret? Are you worrying that someone will find out?
Yeah I'm worried my girlfriend is going to find out about my dirty little secret.
I see that you like older women. It's okay to be attracted to whoever you want, but since you currently have a girlfriend make sure you don't act out on those attractions.
Too late I came into my girlfriend's sister's room and now I'm deep into her
It's not okay to be creeping into someone's bedroom, especially if you're dating their sister. Liking older women is fine, but the way you're acting on it is not okay. You're likely to lose both your girlfriend and any chance with her sister.