That's an interesting one.
I am my partner's second wife and will be the sole beneficiary if he predeceases me. ( He has two children from his first marriage.) He has, however, asked me if I'd continue to keep an eye on his older child - now in his late twenties, whose life and money-earning abilities have been complicated by his not being neurotypical.
I think the logic of his decision is
a) both his two older children will inherit when their mother, who has not remarried, is no longer alive.
b) his third child - our joint child - will, of course, not benefit from a)
c) I have supported him in his career, and assisted him in setting up a business, as well as prioritising looking after all three children. Our money has been considered as 'joint' money rather than 'his' or 'mine'.
I do ponder about what eventual responsibilities I might have about my stepson. However, he is doing quite well, at the moment. I think it might be more a matter of being there to advise and help him, should he need it. His not being neurotypical means there are repsects in which he's not especially good with money. I think I'd just have to think about what my husband might do in any given situation. (He's the sort who asks searching questions about robust plans first, when it's to do with children and money - rather than automatically digging into his pocket for yet another handout.)