I feel your pain. I have two sisters. One has 4 children by two different fathers and has not worked for 30 years due to agoraphobia, ( though she's always posted about attending protests and marches, takes the kids to school and goes out shopping). She's just work shy. Yet my mum idolises her and treats her 4 DC's to everything. I tried to form a close relationship with my neice and nephews, I always took the time to treat them yet gave up this year due to thinking for 20 years I've barely had a thank you from any of them and my sister has never ever given my DD anything.
My other sister and I work hard and only have one DC each. Not only has my mum never had them overnight, or even for a day out, they often never get a mention or thought. For example my DD did really well on her A levels, not even a response like 'well done' let alone a card or present. Years ago, I witnessed my mum walk right past my daughter and didn't recognise her, I was a few feet behind her, she saw me and I said why have you not knowledged my DD, she said "oh I was miles away" and the proceeded to ignore DD! My DD knows that's her gran, but she doesn't call her grandma, nanny, gran or any other iteration, like me, she gave up years ago! She treats my other sister's DS the same way. We all gave up trying years ago. Sometimes there is no logic or reason, I think possibly it is a mental illness and my other sister is weird and maybe just feels more comfortable in that dynamic, than her two other GC's lives where they are both in loving, caring environments with all parents owning houses and working hard.
Personally, my advice; tell her to stop sending anything to your children as it's confusing and upsetting, either start treating them all equally or stay out of their lives.