How do I not let things bother me so much?
Thank you for your question. It is important to remember we chose the person we marry but that person did not chose their biological family (parents, siblings, grandparents). Having difficulty with in-laws is common and can be extremely uncomfortable. The spouse can feel stuck in the middle because they don't know how to navigate interpersonal difficulties between their spouse and parents.
First, I suggest you work on continuing setting boundaries. You set your first boundary when you decided to move out of your sister-in-law's home. Good for you for doing that! It must have been difficult. Now, you work on articulating your boundaries, such as informing your in-laws you are not willing to engage in this type of behavior. If they cross that boundary, you can respond by stating something like, "I'm not doing this with you." Remember, their behavior is not personal. They have probably been acting like this most of their lives, and will likely not change. It is important to not take ownership of other people's behavior. We get to take responsibility for our own responses to other people's behavior. Having this outlook can be empowering because the only person we have control over is ourselves. This can be worked on in therapy. It takes practice, but the result leads to you being free from this negative reaction and having a closer relationship with your spouse. People that behave in such ways as your in-laws attempt to bait people. It is important to not bite! If you don't "bite," they tend to move on to another topic. I'm a visual learner, therefore I like to visualize a form of bait being thrown at me when someone states something with the hope of receiving a reaction from me.
I encourage you to also work on boundaries with your spouse in dealing with future issues with his parents. This can be done in a therapeutic setting as well. Again, remember, we don't chose our families. It is not fair to punish or blame the spouse for their parents' behavior. You two get to determine what you are willing to compromise on and work on in the future to make your marriage stronger without the influence of your in-laws.