I would think if you have a good relationship with your brother and SIL that this was probably not intentional sabotage. I imagine they don't understand low carb and didn't think it over very well. Maybe they thought it meant no sugar. Or thought the amount was so small it really wouldn't hurt. Remember they don't have the knowledge of your plan that you do.

I guess another thing I'd think about is if this is worth being angry over. You've eaten it, it probably won't affect your program at all, and it's over and done with. I'm sure she isn't in the least angry and you're eaten up inside. Let it go now

Losing weight will be a great example to your family. Il'l bet you that they'll be back asking you how you did it and paying attention and getting support from you before too much longer