Thisname it was an insensitive thing to say and you were right to be offended. Although I know she did not mean it and I would not make a huge fuss. But I do think it may be helpful for you to understand why you or others would/might be offended.
If you want to pull her up on it I would do it quietly when no one else is around and just explain that adoption is complex. We all know we might talk about babies as buns in the oven and say stuff like I'm cooking up my baby etc. But the ready meal analogy is (IMHO) very insensitive.
I have a dear friend who said something like, "Adopting is just like having a birth child, except you don't get to pick their name!"
No it's not!
Another friend told me I was lucky as I did not have to potty train my son.
The flip side is that I do not know all his experiences, not only have I missed out on crucial bits of his life so far but he has missed out on having me there for crucial bits of his life and might have missed out important stages that all kids normally go through.
The early experiences he had, including being taken into care, have given him anxieties that he would not have, and have moulded him in a negative way.
It's not the end of the story because I hope I can make up for some of these things but it won't be as easy as eating a ready meal!
I think the reason adopters get offended by flippant comments is because it over simplifies things. You take these fresh natural ingredients and make this or someone else takes these ingredients and does this and that and you get a meal. But everyone's intention is to make a good lasagne or cottage pit etc.
With adoption you are taking a child who someone else has parented badly and let down, and you will have to help them to overcome those early experiences and, ideally, flourish. With a very few exceptions I think this is the case.
I think (so far) adoption is very hard and also amazingly rewarding.
I hvae just helped my son to learn to jump into the swimming pool, I am so proud of him. He is almost 4. My (birth) daughter was having baby swimming classes from age 6 months. It is a very different experience. It is a good experience (a lot of the time) but is parenting a child who has had (perhaps) very difficult experiences.
So trust your instincts when people say things that are offensive, pick your battles in terms of helping people to understand and make sure they do know that those types of comments are not acceptable around your new child (even if they are pre-verbal).
One of my friends kids kept referring to our son as our adopted son. My friends kid is a bit quirky and I know he did not mean any offence, what he was saying was true but I quietly said when we were alone. "He is our son, we adopted him and just call him our son, it doesn't matter how he joined our family." I made a point to tell him when he was alone rather than with our ds or other kids and he was fine. If I had not said anything he could probably have continued to say it thinking it was fine.
We are our children's advocates in things big and small but must do it in a nice way because you never know when you will need your sister to drop everything and run round to prepare a ready meal for you and your family if you are all ill or something!
Good luck.