First of all repent to the Almighty. All sin is equal in God's sight, except the sin against the Holy Spirit. Once you have repented have peace. Do not let Satan continue to plague you with guilt.
Bring your sin out of the darkness and tell your DH. Satan works in the dark, Christ works in the light. Before you do tell your DH pray that the Almighty soften his heart to be able to forgive you. I am sure your DH will appreciate your honesty. We ALL sin and fall short. True character in a person can admit wrong doings and forgive the unforgivable.
An intimate moment could be a touch/kiss/cuddle or full intercourse. Nevertheless, you should apologize to your sister in law too. Let her know it will never happen again and keep your distance, if you feel you are tempted.
I have learned over the years that sometimes a female is easier to talk and relate to about the deep stuff we feel. Sometimes a husband just doesn't get it. Regardless, your DH needs to be the go to for ALL intimate conversations. That is how husband and wife bond emotionally and spiritually.
Remember you are forgiven through Christ's blood, and Christ chose to surround himself with "sinners".
Blessings and you are in my prayers....
A lot of good can come out of this dark valley that may bring you and DH a lot closer together and be able to help out another couple!!!
Use this time to poor yourself into Scripture. God will use this dark time in your life to "grow you up". Trust, trust, trust. God will never forsake you.