All in the Family...

You say she's "very intelligent," and you told her to “just borrow these clothes from her sister instead of buying them.”

What's wrong with this picture?

Sorry, but you are doomed. Sisters, brothers and best friends are the taboos of fantasies.

Find someone else to fantasise about and you might rescue your relationship.

I notice a few interesting things here - you claim you "had better", but once you fantasize about her sister everything is great ?

You are no longer as aroused by her as you used to, and you try to "fix" it by fantasizing about other women ?

First of all, look at the reasons why you "had better". Does this apply to other women, or does this apply to her ? If she has always been crap in bed, why haven't you done something about it already?

Okay, so you are turned on by slutty schoolgirls. I can't say I disapprove, as long as you don't actually act upon your urges. That's what girlfriends and roleplaying is for.

It is also common for older men (and you are close to 30 now) to aspire to younger women, possibly as a way of staving off their midlife crisis to a point where they can actually afford a penis-shaped car like a Porsche.

Again, as always, the secret to success is talking. Talk to her, tell her what you like in sex, tell her what you don't like, and get her to dress like a slutty schoolgirl. Or a nurse, or a nun. Whatever grinds your rod.

And, seriously, don't mention her sister. Tell her that you saw those clothes on her sister and fantasized about your girlfriend in them.
That's an important point - you don't want her to think you want to bone her sister, and you really don't want her to think you're child molesting pervert who takes quick trips to the local high school for underage sluts.

Every relationship has something like this, sex becomes stale and it becomes more work to get really great sex. So now it's time to explore a bit more, get out the whips and chains, fancy costumes, have quickies in church and invite that swinger couple that wanted to hook up with you for some time.

Just ... don't ... mention ... her ... sister !!!

--Does she know seomthingsomething?--

Yes, she does know seomthingsomething! You probably give her sister way too much of your attention. I'll bet you even stare at her when you think nobody's looking.

--She once commented, when I asked her to dress up certain ways, that she could “just borrow these clothes from her sister instead of buying them.” --

Did you describe her sister's last outfit, in great detail? How much time past from seeing the outfit to suggesting to your fiancee?

You might try everything you can to get the sister out of your mind.

Dude. Talk to her. Really. Ask what she likes. Do it for her. Tell her what you like. Ask her to do it.

You need to read Nancy Friday's My Secret Garden. Women have a wide variety of Sexual Fantacies as well. It's allowed, its natural so why can't men do it?

Women have a wide variety of Sexual Fantacies as well. It's allowed, its natural so why can't men do it?

Um ... because women have double standards? Because what's "liberating" for them to do is "degrading" when men do it?

She once commented, when I asked her to dress up certain ways, that she could “just borrow these clothes from her sister instead of buying them.” Does she know something?

Well, even if she does know something, apparently she doesn't know enough -- or care enough -- to actually want to do something about the situation. If she hasn't tried dressing up in trampy clothes even though you've asked her to as a way of improving your sex life then that might indicate the two of you have additional issues to be worked out before you get married.

Threesome has already offered the advice this guy needs to follow:

And, seriously, don't mention her sister. Tell her that you saw those clothes on her sister and fantasized about your girlfriend in them.
That's an important point - you don't want her to think you want to bone her sister, and you really don't want her to think you're child molesting pervert who takes quick trips to the local high school for underage sluts.