@Bonnie12,
Calm down. You are all over the place.
First you feel trapped. Then you feel paranoid that you shouldn't have said something to your sister. And your concerns seem to be only for your affair partner, and not for your husband or your kids, or even for your sister, who you are essentially asking to lie for you (or at least lie by omission, if your husband ever asks her if there is something going on).
BTW, your doctor can lose his license for this kind of crap.
Get thee to counseling. YESTERDAY. And not a referral from this doctor; ask someone in his practice for a referral, or ask a friend if you must. And find out what your next steps are. What you are missing (for real, and not just excitement) in your life, that you feel your husband can't fulfill. Figure out whether you want to leave, or to work on things. And also figure out what to do when your husband and kids find out.
I said
when, not
if. 'Cause they
will.