Wow it sound like a issue that is very common with family whether biological or by marriage. When disagreements with family occur it is difficult to stay objective and not get a meshed in drama that escalated quickly or destructive cycles that just make you more frustrated and upset. Poor boundaries and disrespect can be hard to resolve and when gossiping or I’ll intending outsiders get involved the situation can quickly become a dispute that can deteriorate strong relationships or worsen relationships that weren’t healthy to begin creating animosity that never resolves. To save yourself some sanity I would first create boundaries to stick to no matter if the person finds them insulting or irrational. I would then remind myself that you have to try to let go control and expectations on how they should be have ” you can’t change them, their words or action but you can Change your reaction by trying to keep your cool” finally if you have already tried a sit down to discuss how you feel only to be rebuffed you might think about retreating for a while until you feel differently or you can Stretegize new tactics for positive interaction. It’s ok to establish boundaries its probably not ok to attack the person or engage in toxic interaction or visit if you know that it won’t end positively.keep in mind your family member has flaws that are more about them than you it is not your fault and it’s not your job to fix them or people please if their is no pleasing them. People pleasing is often ineffective and exhausting not to mention reenforces powerlessness and resentment if the person is not gracious. Finally picking your battles,not keeping score can save you a lot of grief as well as empathy in some situations can at least help you try to understand where the persons coming from and remind you the person is a person and is human that might make mistakes and not have all the answers themselves. At best if you don’t get resolution forgive and either move on or try to slowly build up your relationship trust and dynamic without grudges or past resentments.who knows they may surprise you , you may even get an apology