Have been doing cakes for over 20 years, have 4 younger sisters and 3 SIL and all are always amazed at the cakes I make. Have only done 5 wedding and quit that - to stressful for me. If anyone didn't like what I was doing I would tell them that they could do the next cake, I would supply everything for them and see what they could come up with by way of decorating as I am a very easy going person. Let the SIL try to even half do what you did. Your cakes are beautiful and son and DIL will remember your work/love forever.
Your cake is beautiful - be proud!!
I also have to deal with such criticism - from my mother!! She is always telling me "don't let anything out of your house if it isn't perfect, you don't want to ruin your name" (I don't do this for business, don't plan on until I take a Wilton course). But in public, she is all the proud mom, showing off my stuff, even asking me to make things when she wants to send stuff to other people... My siblings and DH help me brush it all off, she's a perfectionist in everything.. so keep your chin up, ignore such people!! They don't know what it takes to decorate cakes 
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