I am married and me and my husband are devoted Christians. That makes this even more difficult for me to comprehend.
Since that is the case, I will answer your post in accordance with the scriptures. Note, I said with reference to the scriptures, not church tradition.
As it happens, I seem to have done something that I believe might be sinful and I am feeling dreadful about it at the moment. My husband has a sister whom I have known a long time and is very close to us both. ...
I can't really stop thinking about this. What should I do? Do I ask God for help?
When we were sleeping in the cottage I woke up in the night and me and her (the husband's sister) were all tangled up and arms around each other. This is one of those things you just can't explain but she was awake too and was just looking at me. After awhile she started to move herself against me and it bagan to feel nice and I guess we got caught in the moment. We did not kiss or anything like that. I'm not sure exactly how long it lasted but after and during it evoked such feelings in me that should only be happening when intimate with my husband....I guess the description of us rubbing against each other is the most accurate and the end result being an orgasm. From her trembling breathing and voices she made I guess she experienced similar feelings as me. I am ashamed to admit that while the feeling was similar I most likely have not ever had an orgasm while being intimate with my husband. At least not that I remember.
All this talk about gays and bi-sexuals and normal behavior is so confusing. I was told that gays go to hell as a child unless they go to therapy to get cured. I am afraid to tell about this to anyone least my husband as he will surely leave me if he thinks I am not normal.
#1 What you did, or I should say what your SIL did, is not forbidden by the scriptures. In fact, it's not even mentioned in the list of sexual sins in the Bible (Torah-Five Books of Moses - AKA "THE LAW"). However, homosexual sex between men is condemned in the Torah. Yes, the Bible has a double standard.
So the actual act in and of itself is not called out as sin in the Bible's sin list. In the last 150 years or so, people have tried to claim that Romans 1:26 condemns female/female action when it says: "For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature."
Since nothing in the Bible anywhere else says two girls is a sin and the context of the passage is a discussion of pagan gods and myths, in which bestiality plays a big part, bestiality is most likely what Paul is referencing. Bestiality, for both man and woman, is in fact a sin according to Leviticus. So, that's the only passage one can point to, even erroneously, as being a denunciation of girl/girl sexual activity. Previously, that same passage was used as a denunciation of anal sex. In context of a discussion of pagan gods and their propensity for bestiality with human females, it fits nicely with the Old Testament list of sexual sins.
The people who wish to use Romans 1:26 as an anti-lesbian sex verse support this modern interpretation with Romans 1:27 and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
Now that is clearly male homosexuality, but male homosexuality is already on the sin list in Leviticus. Remember, the Biblical standard for behavior is not the same for men and women.
#2 Biblically, and legally, what you did is not adultery. Exodus defines adultery as a man copulating with another man's wife, and many legal jurisdictions still do. There is no way for your SIL to adulterate your husband's seed and produce bastards, so adultery it is not. No man, no seed, no adultery.
So, there is no sexual sin committed, according to the actual biblical definitions of sexual sin.
Now, as for the rest of it, if you know your husband is going to object to this, don't do it again. There is no need to confess to it and there is no need to worry about it. It happened, you didn't plan it. No sin was committed. You don't even need to bring up the subject again with your SIL.
However, if you're going to keep this up, you need to get your husband's permission, since you as a Christian woman, are under his headship. I Peter 3: 5,6 "For this is how the holy women who hoped in God used to adorn themselves, by submitting to their own husbands, as Sarah obeyed Abraham, calling him lord."