I've had this condition since my early teens. My reason for it - it stems from social anxiety, or fear of being judged. Even though I know it's a perfectly natural thing to do, I'm 38 now and I still find it hard to go when people are around. I remember my honeymoon being a complete torture. I used to wait for my husband to fall asleep and then try and go. To this day I still haven't told anyone about it, except for a therapist that I saw a year ago when I was doing Cognitive Behaviour therapy for social anxiety. Basically what you have to do is start of small then work your way up the ladder to your most feared situation. For eg. if having a bowel motion at home when your sister is present is less uncomfortable then say your boyfriend being around, then you could start off working on that fear first. You could go to the bathroom, grab some toilet paper, wet it under the tap and scrunch into a ball, wait a few minutes and then throw it into the toilet (to resemble a bowel motion falling into the toilet) and then flush. Go outside and look at the reaction on your sister's face - she will probably look at you as if it was no big deal that you went to the toilet and did the normal thing. Do this several times when she is in the house and then work on actually having a real bowel motion when your sister is in the house etc. I haven't tried these techniques yet due to my own preoccupation with other parts of my social anxiety that I have to work on, but give this a try and post back. P.S. I'm new to this site. Good luck.