There is already a thread about Sister-in-laws in the relationship forum.
http://www.lushstories.com/forum/yaf_postst18900_Sisterinlaw.aspx However, this is sex tip forum so I guess you just want to share your fantasy. So yes, I also had a little fantasy about me, my wife and her younger sister. I even talked with my wife about it, but we both agree this is something I should keep in my fantasy world. It's just too much risk to jeoperdize our marriage.
This maybe totally out of my business, but I'm not sure about the idea of posting your sis-in-law or mother-lin-law's erotic pictures in your profile. Unless you got their permission to do so, otherwise I'd say it's a little disrespectful.
Just my 2 cents.
Welcome to visit my tumblr page at
http://HK4167.tumblr.com for erotic arts. Or
http://slice-of-moment.tumblr.com for non-erotic arts.