It seems that so many on here feel the same way as you do and the more that you do, the less you feel appreciated.
One of my nieces has always been offhand with me.I came back from overseas when she was 3, so my sister had already been with her from birth and formed a relationship with her.
Despite whatever I did, she has always kept me at arms length.
When she lived in Cornwall, she was insistent that I went to stay with her and her family, but instead of spending time with her - going out for meals, to the local ice cream parlour, pub etc like she did with my sister and her family, I was treated as if I was staying at a B&B!
I left there, wondering why she had bothered to invite me!
Since then, things are really no different between us, whatever I try.
She even moved house and didn't tell me for several months , let alone tell me the new address- I only found out after sending a birthday card and having it returned by Royal Mail!
I am never sure if people, especially children realise how hurtful they can be.
Personally, I would cut back on the extra gifts and have no problem with explaining why to the parents as well as commenting on the fact that you were left to have Christmas on your own - they obviously assumed that, despite you acting as unpaid childcare over time, that you really enjoyed your own company while they were having a happy family time.
I would hope that they will feel horrified at how you feel and take steps to include you in future, but I'm afraid that things may never change and now that socialising is becoming more possible, that you expand your social circle - there are many people I am sure that will fill that gap, who are in a similar situation to you.