Dealing with my SO's Enabling Family Members

I have been with my partner for a little over a year. In that year, we have had a lot of struggles, which I originally posted about here :

He is undiagnosed as of yet, but it's very clear he struggles with ADHD, to both of us (and his teachers growing up, some family members, etc.)

In spite of the problems his irresponsibility have wrought on our relationship, we have been working together to move forward and make some changes in the last few months- and I'm seeing a lot of improvement! However, I've also recently noticed something that may be detrimental to our efforts to get him to be more of an independent, responsible adult; my SO's sister is a very organized, "mom" sort of person. Their mother growing up (I'm told) was very busy and not always totally "on-point" with some of the typical mom stuff, so I suppose his sister took over in some ways and took responsibility for my SO and his younger brother.

When my SO told me stuff like that, I thought it was sweet that his sis is so caring and helpful. But as we've gotten closer and I get to know his family better, I'm seeing a bit of a dark side of it as well- and the things that he sees as "helpful," or "organized," I can't really see as anything but enabling.

There's obvious stuff- like lending him money, going out of her way to plan rides and things for him when he hasn't planned properly, etc. But it goes farther than that as well-

As an example- whenever there is a family event for someone's birthday approaching, sis will start a message thread with my SO, his brother, and usually myself and his brother's girlfriend as well. These threads are about the plans for the event, who will bring what, and contributions to a gift. However, what this basically becomes is myself, sis, and the other girlfriend planning these things while these two guys contribute basically nothing. The boys often don't have money for gifts, so I (and likely bro's girlfriend) eventually contribute their part (both because I care about the person having the birthday or whatever, but also because it's awkward to be in the situation), and we also end up buying and preparing any food, etc. for the event.

I do not want to participate in this kind of thing, because I feel like it should be on my SO, entirely, to a) remember the important occasions in his family's life b) plan and purchase any gift he will be giving them c) figure out what will need to be brought to the event, if anything, and make it happen himself.

I have never expected anyone in my life to buy my dad a birthday gift and plan how I'm getting to his party, and bring my contribution to his birthday meal. EVER.
And as far as I'm concerned, doing it this way is only enabling him to not take responsibility for his own part in these occasions. If he forgets, then he experiences the discomfort of his forgetting- maybe next time he doesn't forget. If he doesn't save money and get a gift- then he experiences the result, if any, and maybe in the future he chooses to act differently.

This approach does not encourage a different result, or encourage him or his brother to man up, act responsibly, and deal with the consequences when they don't.

I'm planning to talk about it with my SO- but how can I discuss this with her without her getting angry at me, or defensive, or basically just messing up what is still a fairly new friendship between her and I? I know she is doing this out of caring and her intentions are 100% based in love. But from what I see, it's encouraging the negative stuff more than the positive.
Any tips, stories, anecdotes etc. of similar situations would be greatly appreciated.