Ilan,
What a pain filled post. My heart really goes out to everyone involved.
Your letter is such a clear reminder to me that a persons actions affect so many people beyond our immediate selves. Like a pebble in the water, the ripples spread in an ever widening circle. Those closest to the 'splash' are affected the most; your sister, brother-in-law, (children?). Next level would be family, then friends, work colleagues etc.
But within every trial are positive lessons to be learnt. I see so clearly how family conscious you are. You care so much about your Mother's opinion regarding your addiction, and your sisters pain touches you deeply. Sometimes it takes hard times like this for us to remember how important certain people are to us.
I see that you have also learnt the bases for a good marriage.
Just remember that when you get married, your wife takes priority over everyone else especially your parents.
No one asks for pain and hard times, but once Hashem has seen fit to send them anyway, our job is to take the lessons and move forward with them. You will now be a much more sensitive husband when the time comes.
As for the addiction, perhaps thoughts of your sister could be of help in restraining. Each time you wish to view an inappropriate site, try hold back. Talk to Hashem and try make a 'bargain': Hashem, in the merit of me holding back at this moment, please help ease my sister's pain. Perhaps connecting your needs to something larger than yourself will give you that extra burst of energy to restrain
veels geluk