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I'll do my best to try to help you with this issue. If you're available for chat, lets get started.
I'm so sorry to hear about this and from what you've told me here everything you have done has been appropriate.
It's easy for parents to be critical of their children, especially step-children. The next generation never does what they did (information changes, science changes, advise changes, the world changes) and they find that frustrating. It's generally a lack of information on their part and a failure to understand how science and parenting change over years.
Your reaction is certainly reasonable and I think you're right to feel as though you're parenting has been questioned.
It's a tough place for you. I'm sure it puts a strain on your relationship with your father and this time of year it makes things especially difficult.
If you really want to get past I would suggest that you set a time with her (without the kids in tow so you can focus on the discussion) and calmly explain to her how her comment made you feel. Explain to her that it was hurtful to you and that it gave you the impression that she didn't think you were a good mother. Explain to her that it didn't occur to you that she would expect to know details of her evaluations but that you would be happy to share that information with her in the future. All she has to do is ask. Hopefully being able to speak with each other calmly (that's the hardest part---I tend to start crying :o) ) will help close the issue. It would be nice if she apologized to you, but you should be prepared that that may not happen. If she bristles at the discussion or begins to attack you again you can feel that you've done your part to mend the relationship and the ball is back in her court. At least you can feel like you've tried.
I hope that's helpful. Be sure to reply to me if you have questions or I've left anything unanswered. I hope things work out for you and that you two can get past this conflict.