I think your feelings are valid and reasonable in both situations. For your SIL, I'm going off of what I've heard from a coworker who had a double mastectomy with reconstruction: she said the thing that really shocked her was just how tired she was. She wasn't able to do much cooking or housework even a few weeks out. We all pitched in to send chocolates and a gift card for doggy daycare and she said that was actually super helpful. So if you do a care package, maybe a meal or something? A heated blanket? Not sure how, but anything that takes any burden off of them but lets you avoid her mother. I hope that makes sense.
For the vacation/taxes, firmly stick to PDs suggestion of getting it done the weekend before, or not at all. Even if you can afford to lose $80, it's important for you guys to get away. Don't give up your vacation! I know your in laws have had medical issues but they obviously know when the taxes are due and that they need to get done. Feel free to remind them every day when your deadline is, and if they miss it, PDs siblings can deal with it.

In practice I absolutely suck at setting healthy boundaries, but I can tell you that the lines you want to draw are perfectly fine.

I have a friend who has had a mastectomy twice (first one was 25 plus years ago and back then they only did one side unless both had cancer) and the second one less than a year ago. Her favorite thing while recovering the second time was having a heated throw to snuggle up with and a heating pad for her back or what ever was hurting (she also has fibro etc) The first type of cancer she had only 5 percent of people are still living after 5 years, shes still cancer free from that cancer over 25 years later. The cancer she just had removed was the easiest kind to get rid of into remission. Both scary. She is on oral chemo for 5 years which she does struggle with side effects with. Also she was so run down that it really helped when people gave her gifts of some of her favorite snacks and chocolate and healthy snacks as well. She also loves coffee so that was one thing she would enjoy while resting with her heated throw. Also is there any hobbies she has? Or does she like doing puzzles or puzzle books etc or reading? Activities she can do while resting in a recliner or on the couch are also good or maybe a pair of very comfy pjs or lounge wear!

I say keep your vacation and go no matter what! They know you have plans and when you are able to help. Yes they have health issues but YOUR mental AND physical health are just as important as theirs! You have MORE than earned your vacation! at least 95% of the time mom does understand if i have to cancel coming on day or change the time or if i cant commit to something due to pain, right now i do push myself more to help her due to how sick she is from that dang deer tick parasite in her blood and dealing with c diff issues but i also know when to stop and rest! Which reminds me i need to make cupcakes before 1 pm tomorrow her two *puppies* turned 3 this week, one today and one this past tuesday they are cousins, their moms are twins :) They are raised like twins as well! We are having a puppy party with moms 5 dogs and our 1 tomorrow and another friend might come of moms too!

Great ideas ladies and thanks for your input. She likes knitting and reading, do I could maybe find a nice thick book for her, maybe even a small series. I like the food ideas too. Lounge wear is a good idea too; even though she telecommutes they told her to take FMLA because it will be at least 8 weeks before she can work an 8 hour day.

And thank you also for the feedback regarding vacation. We are going camping and only camp when it's not hot due to the humidity effecting my migraines.

I think that a short term meal service plan or a maid service might be helpful.

And, you should go on vacation as scheduled. They've been told repeatedly about the deadline. You deserve and need some time to yourself. No reason to feel guilty when they don't want to cooperate. Go and have fun.

Go on vacation! Please! I'm schedued for one and I'll give it up onlly for major disaster. It's self-care, PuppyMom, you need it!

I agree with the other suggestions, especially a series of books she can read, maybe each one relatively short (mystery series often are that way). Comfy gear is essential, but maybe a gift card so she can get what she wants online?

HUGS to you.