I thinkyou have a DM problem, OP, and not a DSis one.
IMO you need to point out to your mother that DSis is now pushing 30, well and truly an adult and not a child, and that as you're both adults, where are your presents and cards? When she was little, and you were grown up, fair enough, but now? You could always ask if she's aware that now DSis has grown up the expectation's stopped being age approproriate and morphed into inadvertent favouritism, and she compounds that by seeking to police your relationship with your sister to her benefit and your detriment.
IMO the odd row can clear the air and shift things in better directions, in an otherwise calm and non-confrontational relationship, as long as you don't get into mud-slinging, or say unfair things that can't be taken back.
My DH's parents treated him as a small child well into his 30s. Would turn up at workplaces to take fricking photos! Would email him, imperiously telling him to provide his CV so they could consider how to improve it. My especial favourite was when they said he needed to 'make his case' on why uPVC windows weren't a good idea for our Victorian terrace, instead of sashes. Our house. And while it was kind of them to want to pay for new windows, the way it was framed - an announcement that we we going to have windows they chose to put in, and we weren't even asked - took my breath away. At the time, DH wasn't yet seeing it as weird and we had a row because he was worried refusal would upset them. (I said devaluing our house would upset me, and he lived with me, so who did he most want to upset!) But over time, as he took on more and more responsibility at work and we had our own kids, he started to find it grating and then genuinely unbearable, and constant tactful managing of this so that they didn't actually know what was happening in his life, and couldn't take charge, beyond his patience.
He pushed it one year to a confrontation, which was bloody horrible, frankly, but since then, he has been ever closer to his dad and step--mum, both of whom now treat him as the grown man (his stepmum always having thought their attitude to him really weird, it turned out), and less and less close to his mum, who still seems to feel personally betrayed that he had the effrontery to grow up. But she doesn't try to control him any more, either, because he made it clear he wouldn't tolerate it, and held that line through a year when she wouldn't talk to him at all. She had no option but acceptance. Now, they have a much better relationship, in truth, though she probably won't see it that way. He doesn't dread seeing her, or resent her, as he did.
Family dynamics suit people, so they push back on any attempt to alter them. If you want this one to change, you need to be calm, dispassionate, and very clear: there are two adult women, no children, and you are expected to cater to the other while the other is not expected to consider you at all. That's ridiculous, unfair, and wrong.
Being PA and silently resentful won't shift this. Being calm, adult and consistent in refusing to accept this lunacy and just disengaging will. It's amazing how fast people adapt to new status quos when there's no alternative, and to be fair to your sister she's probably never stopped to think about it from any viewpoint than your mum's. She's been brought up in this role, with this expectation, and if you calmly point out that if two adult women are in the picture then the rational position is mutuality, either gifting one another or not at all, then a penny may drop for her. And if it does, you can have a better relationship.
Right now, your mother has set up a dynamic that's guaranteed to breed dislike and resentment, and you're the only person who has any chance of shifting that because it suits everyone else too well to stay in role... your sister loses any hope of a genuine relationship with you of course, but that's a hidden cost, right now.