Psalm 51 is the best confession passage in scripture. Verse 4 says that David (who wrote the Psalm) confesses that his sin was against the Lord only.
There is a general statement about confessing our sins one to another... which IMHO is more a statement about not being a hypocrite and acting as if we don't sin at all... you already said you confessed it to your sister.
David sinned against Bathsheba (even if she may have had no qualm about bathing on the roof of her house in view of the palace), he sinned against Uriah in the process, he had Uriah murdered after his plan to cover up his sin failed. He got Joab to help in this plot. The child born in sin died, as did Amnon and Absalom, and his daughter Tamar was raped by her half brother.
Psalm 51:4 (AV)
4 Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest.
Now this may also be because the whole world knows (or has biblical access to all of the lewd details... I don't know. I do know we should all study 1 John 1:5 - 1 John 2:17 with regard to our sins that we are already forgiven for (and thus we are only in need of getting back in touch with that forgiveness which our sins and guilt alienate us from [feelings wise]).
Consult the Lord / have people over in Prayer Requests here pray for you.
He may want you to tell your husband.
Any who saw your brother-in-law's actions, your sister who you confided in, and your brother-in-law (I suspect remembers and is just covering his guilt) all know what happened.
You said you told your husband about the incident and most likely he will confront his brother about it eventually.
Your dilemma is in feeling guilty over being human (there are many things we should not say or think or do that we like, love, or get turned on by). This is a major reason God gave us the commandments not to indulge in them. But we live in a world that indulges them, flaunts them, encourages them, or where it is acceptable to become uninhibited through excess drinking or drugs... we are animals by nature. From the time we are infants we are conditioned, trained, disciplined not to give into our baser urges (beginning as early as toilet training).
This is what we must remind ourselves of daily since we tend to forget it and often deceive ourselves.
We are not good by nature.
God living in us and among us is the only good there actually is.
As Tim Allen said in a Last Man Standing script once to his "daughter" Mandy, "If you tell Kyle you kissed another guy, you may feel better about what you did but you will make Kyle feel terrible."
She replied, "But I feel just awful about it."
He responded, "Perhaps you should."
Maybe it's a lesson to keep it from happening again.
Lastly (and this is a big one):
The Bible speaks to the Lord's grace being greater than our consciences. Sometimes we can find it impossible to forgive ourselves. And the devil can bring up any number of our past sins to guilt trip us into th insane asylum if we let him. Don't! I can't recall the pastor who said it, but it was inspired. He said, "Whenever the devil reminds you of your past... remind him of his future."
I don't believe most can go into exorcism type confrontations with demons, which is another topic, so I do not encourage that. But mentioning (aloud*) that death and hell and the serpent will be thrown into the lake of fire where the beast and the false prophet are to suffer forever and ever shouldn't cause any trouble... its quoting scripture (Revelation 20) and it [should] stop any 'assisted' feelings of guilt.
* aloud because the devil and demons cannot read your mind
Hope this helps. And I have prayed for you, sister.