Making soap and teaching soap making are two entirely different things. Teaching takes a LOT of time and energy to do it well, and if your heart isn't 100% into this idea, then don't. Saying yes to this is a good way to set yourself up for resentment and hard feelings. Better your SIL and her friend be unhappy than you be unhappy, right?
There is no "Iowa nice" way of saying "no" without actually using the word "no" in a situation like this. Yes, you can be polite about it, but I'm with Brewer George -- there's no need to make long explanations, half truths, and torturous excuses. Just say something to the effect of "no, it's not something you want to do" and leave it at that.
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I have given presentations around my local community about the history of sleigh bells (which is what I work with in my day job). More and more people, complete strangers, were wanting me to give a 20-30 minute presentation ... usually evenings or weekends, unpaid, at a location of their choice. I got in the habit of saying "yes" because ... well ... it's the Iowa Nice thing to do, right?
I realized my heart wasn't into it after doing about 3 of these events this past winter, all scheduled in the few hectic weeks of our busy holiday season. I was just doing it because people asked me to, not because I got much out of it. Enough already.
The next time an utter stranger called me about giving a presentation to some social group she belonged to, I politely but briefly explained that I had recently decided to not give presentations any more. She blurted out in annoyance, "But if
you won't do this, then what am
I supposed to do NOW?" And I said I didn't know and I repeated the statement that I was not going to give these presentations any more. (I really wanted to say "That's not my problem, laydee!" but I didn't.)
She tried a couple more times to push me into saying yes, but I just politely repeated something to the effect that "I appreciate your concerns, but I am not going to give these presentations anymore." When she finally realized I was not going to sing her tune ... she hung up on me. So I learned that saying "no" sometimes can have awkward and uncomfortable repercussions, but I feel far better about sticking to my guns than I would have felt if I'd caved, despite having to endure that person's rudeness. The practice I got with her has been good -- I got yet another request recently and I found it was much easier to give my polite refusal.
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PS: Madison, you say? Maybe these folks will offer their class again:
https://www.facebook.com/events/245092749255059/