I like the way your thinking has evolved over the course of this thread so far.
Warning: Insomnia fueled, somewhat related rambling ahead. Feel free to ignore.
Is your SO likely to inherit anything from his parents? If so, he could earmark those funds for his siblings. Another idea for him explore is to buy a cheap term life insurance policy naming the sibs as the beneficiaries. You could also buy insurance and designate your sister as sole beneficiary.
I have followed that Inheritance thread from the beginning and have my own stories to tell since my parents died. I was single for most of my adult life and planned on leaving the bulk of my estate to my siblings. I also have no children of my own.
Oh boy, once I went through the process of settling my parent's estate as co-executor, I totally changed my mind. I have five siblings. One of them stole from my parents and then frittered away her (reduced by her own larceny) inheritance. Do I want to let her do that with my hard-earned money? NO! Do I want to cut her out completely and leave my surviving siblings to deal with her bullshit at not getting her "fair" share? Do direct her portion to her kids? Will my other siblings even need the money by the time I croak? I have another sister who has weird (to me) ideas about investments and money management. Therefore, despite earning far more than I ever did, they have less. Do I want to give my hard earned money to their causes? Hell, no. OTOH, what do I care? I'll be dead. Finally, five is a big divisor. Whatever amount of money I have left, dividing by five makes the remainder comparatively small.
When we wed, DH and I made each other our beneficiaries of our assets and that's about as far as we got in our planning. Last week, I discovered that one account still has two of my sibs as beneficiaries. I need to change that. I mentioned it to DH and it sparked an interesting conversation. He asked me if he thought any of my siblings were planning to leave any part of their estates to the other siblings (including me). I don't know for sure, but I'm pretty inclined to think they have not. As I am the oldest, my default is to want to "take care" of them. Maybe that's a paradigm I need to shift. Leaving them money won't make them think any better of me when I'm gone, nor will my remaining estate, once divided, be enough to materially improve their lives.
It's a lot to think about.