Whatever your choice is..
I'm only offering advice- not fighting- cause I've seen guys go down in flames in real life over issues like this and other similar ones that tore families apart...
I don't know you, and was only answering your request for advice- it may not be the answer you wanted to hear at that moment, but it came from the heart and experience. It's always easier to see an issue when you're not the one in the middle of it. I've seen this happen before. It always ends badly. I would be doing you an injustice if I simply agreed with you to "fit in" and make you happy.
Fantasy is great. But we're talking real life now- not fantasy. Tickling your Brothers wife and trying to tell yourself it's "ok" or that it's "not sexual" doesn't change the fact that it's crossing the moral line. I don't know if you posted here to convince yourself or just get encouragement from anyone who'll agree- but agreeing to egg you on doesn't make it right either. I used to have a good friend in tickling, who many years back a had a nice wife- and she was even into this for his pleasure to some degree. I was going to bring them to a fair. But in a similar situation, he screwed it all up and lost it all. Small passing tickles- with any hidden passion- turn into more.
But I have the sad feeling that no matter what anyone says, you'll try to find a way to defend your choice to tickle your Brother's wife with comments like "it's not sexual" or "it's not over the line" or "its just tickling".
First off, please don't let your hormones rule you on this issue: it IS sexual. It arouses you- it's not a hobby you learned in boyscouts to merely pass the time. Honestly look where you are right now. You're here, on a sexual Fetish board. Would you show this board to your Brother? or her? If not, its a hidden sexual desire. If you try to say tickling (as you described her) "the finest women you've ever seen with the finest feet" isn't arousing to you in the slightest... well....what can I say? Look at your own words, that's all.
Secondly, there IS a line with tickling- I've met few lately who know where it is, or when they've stepped over it until it's too late because the lines keep shifting. There's no such thing anymore as "it's just tickling" in this PC world. If you think there is, go out and tickle a minor, or someone against their will, and then tell the judge that "its only tickling" and you'll learn a fast hard lesson about these lines.
Frankly with all the beautiful SINGLE women in this world, and with your intelligence, I hope you set your sights elsewhere. But ultimately it's up to you and I wish you the best with whatever your decision. Please think it out.
