Amy. . .I'm betting that if you think about it, you will find that nursing isn't the only thing your sister-in-law has a bad attitude about. She's just more vocal about nursing because it's something that she knows very well, so she can let it rip when she decides to go off. In my many years of living, especially with a mother who was very negative, I've learned that negativity is an attitude. The subjects people use to express their negativity are just tools. If your sister-in-law didn't have you and the subject of nursing to rag on, it would be another subject with another person that would hold their attention, believe me. The purpose is totally self-serving on the part of person doing the complaining. They need an outlet to express their frustrations and feeling of inadequacy. There's a psychological issue of self-esteem going on here as well. That is what is really hiding underneath all crap they are dishing out. It's also very similar to the negative world that depressed individuals live in.
You have to be cordial and tolerate your sister-in-law and the people you work with. My advice is to just be aware that she is like this and keep in mind that this is something she does to boost her own ego and self-esteem in order to feel better. I know that sounds bizarre. It's dysfunctional. She's not fully cognizant that she is hurting you in the process, which is part of the problem for you. I have a sister-in-law who probably is just as bad, but can't trash nursing because she isn't a nurse and doesn't know anything about the profession. However, in her world she will bad mouth a whole variety of things. I'm not a favorite of hers because we don't have many subjects in common between us so I'm not much of a captive audience for her. I won't engage her in an argument either. Isn't that a shame? In their own dysfunctional way, people who do this are looking for a momentary feeling of being a "star" to get attention and dominating a conversation. Like I said, dysfunctional.
You are going to run into a lot of people like this over the years. It's sad when you are working shoulder to shoulder with them and they are standing there trashing the very profession that puts food on their tables and money in their pockets. Rabid little dogs that will bite you is that image in my mind which reinforces the idea that there is something sick about it. If you have opportunities to read up on psychological behavior some of this will be explained and make more sense to you (I'm not saying it makes it right). You can't necessarily change their behavior, but it affects your own reaction to it. Try to find friends and surround yourself with people who have positive attitudes that stroke your good feelings about nursing. They are also living in the world. Hang out with them and you will feel much differently.