It can be hard when to do this when someone else pushes your buttons. But it is important because it is your reaction that defines whether or not she feels she can keep going in the same direction with you. Some of the things to consider include:  Stay silent and there's a risk she just thinks you're dumb, awed by her or chewed up with resentment. Take your pick, she's probably happy to think you're feeling all three. And she'll use your silence to keep putting across her point of view at the expense of yours. If you're grinning and bearing it, you're likely turning into a doormat. Argue and she probably thinks her brother/sister has married an angry, resentful and bitter so-and-so who hates her and will do anything to come between her and her brother/sister. You may feel as if you're defending yourself but to her, it's about you not caring what she thinks and possibly even about putting her down. This doesn't mean there isn't room for disagreement; it just means that the manner in which you realign her understanding must be done with care. State the facts about matters that she pressures you about, firmly but politely, and avoid being emotional into the bargain. If you state things simply, stick to the facts and avoid making it into an issue about her, she has few places to run. Be aware that she may continue to resent you for speaking your mind in an assertive and self-effective way but this shouldn't stop you from clarifying your position. Ultimately, she has to respect someone who doesn't argue, lose their temper or bite their tongue but instead makes it absolutely clear where the boundaries exist. And even if she doesn't everyone else will plainly see that you are the cooler head in the room. For example, let's say your daughter Sheila has been running outside and has fallen over. Your S-I-L insists that she needs to see a doctor or something terrible might happen. You are quite sure nothing of the sort will occur and you know you're a good parent but S-I-L keeps badgering you, upping the intensity of all the bad things that will happen if you fail to follow her advice. Offer your S-I-L a calmly spoken "That's very kind of you to notice that Sheila has a bruised knee but I am thoroughly satisfied that Sheila is going to be all right; this happens all the time and is a part of the way she learns to cope with the great outdoors. She does not need to see the doctor." And that's the end of it, no need to enter into any further discussion. If S-I-L keeps trying, smile and change the subject; refuse to re-engage on the matter.
انتبهي لأفعالك أولًا. ضعي بعض الحدود في التعامل.