I think you will find out that that is fairly normal.
She definitely did not want to hurt you else she would have said to to your face. But that doesnt change the fact that she does feel that way. My sisters and I are easily caught into this trap as well. Its just another form of keyhole judging, only we have a window sized space to look through - still doesnt show us everything.
Also not living a day in their shoes can cause us to feel morally superior.
Turn it around on her. Find out what is driving her comments and explain to her any underlying issues that she is completely unaware of and look at it as constructive criticism - that maybe from the outside she is seeing something you missed. Sometimes I have to point out to my DH when he is being inconsistent with the kids and we dont tend to notice sometimes.
Certainly if you do not want to open your house to be judged and ridiculed in email, don't invite her back, but she does love you and is probably extremely upset she did it now seeing how you found out.
Our sisters were the first people we ever had to compete with - and its hard to shake that.

