If you know that your sister has IBS, maybe figure out her diet and try to match it? My doctor thought I may have a touch of IBS and suggested I give up coffee, chocolate, high fat foods, soda and the list goes on... it's not pretty. As long as I can control it, I'm not giving up the good stuff.
I understand about not wanting to spend a lot of money only to be told something simple is wrong. I spent over $300 dollars out of pocket to figure out what I'm allergic to, and found I don't have a classic IgE response to anything. What do I have? Non allergic rhinitis or allergy like sypmtoms but not true allergies and guess how it's treated---with allergy medication!
If you had a colonoscopy done, it would be a relief to find out what exactly is going on with your intestines. How expensive are they?

I look forward to the day I get mine straightened out too!
Seriously, it works for me.