Have you and your SIL been close before this? I agree with most of the others that you should explain to her how much it costs you to buy the ingredients and how long it takes you to actually decorate it.
I may not charge a lot, but all my friends and family pay what I ask for cakes. My friend just asked me the other day how I could afford to sell her a castle cake w/ 3 layers for only $22. I just said because it was my first time trying it and because it was for her. I always give her a discount because she orders so many cakes from me and I have several standing orders for her to come in the next couple of months.
If you and SIL have never been close this may be why she acted that way. If you have.........maybe it was a bad day for her. You can do your best to keep family harmony, but you should not be taken advantage of.
Tabby
I have an aquaintance that is known for waiting until the last minute to do anything. One nite at a party, she stopped me to tell me she needed a cake for the next day. C'mon, I'm at a party. So I took out a slip of paper and wrote down the telephone number to the local Wal-mart and told her to call when she got home to leave the specifics.
Duh--she didn't know it was Wally World's telephone number. The next time I saw her she asked why I gave her that number. I politely told her there was no way I could have done a cake for her in that short of time but Wal-mart could have. And besides, I told her, I have a $25.00 last minute fee for anything ordered less than 24 hours.
LMAO... This is classic!!!!!!!
I almost needed depends when I read this.... LOL too funny.
You're not wrong to charge for your costs and time. I think your relative is a bit spoiled and self-centered. 
If you can't say no to her-I think your sister in law should buy everything you need for the cake and then you do all the work as a gift. Sounds like a deal instead of buying a gift. LOL.
Right now I'm just thankful my husband doesn't have any brothers or sisters!!
Um, hi, I'm only related to you via via, but see I'm going to this party for someone you don't know. Think you could get them a present for me? yeh, I'm too lazy to pick it out myself, and too cheap to pay for it. But I'd love to get in good with this person at your expense.
Hmmm, ok, I'm preaching to the choir here. But seriously, if it were anything other than cake, even your sil would probably be shocked at such behavior!
mac, sides splitting.
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