There are two reasons not to put your feelings in writing. One, it makes you appear weak, and two, it gives your in-laws something concrete to criticise, and they will nitpick every statement and comma in your letter. Like the first of the ‘Three Little Pigs’, you’ve been living in a house of straw. It is easy to describe your disposition as motivated by kindness, but in fact, it is motivated by fear. You think writing a letter is like building a house of sticks, but the true solution is to build a house of bricks.
Your sister-in-law and her mother didn’t tell you what their game-plan is. You don’t need to tell them your game-plan, or what you know. At your door or on the phone, all you need is a simple, curt refusal. If you must use an excuse, tell them the upheaval of the divorce makes you unwilling to get together.
Your new life is beginning; you are going to be master of your world. In small steps and safe situations, put your needs first. Utter your opinions, beliefs, and feelings without apology. Slowly expand your comfort zone. The more you practise this, the easier it will get.
Your sister-in-law and her mother can huff and puff all they want, but they are not entitled to take advantage of you. No, they aren’t; not by the hair on your chinny-chin-chin.