I wouldn't use Christmas gifts to judge whether she loves you or not.
You admit you go overboard on her kids and will certainly have their gratitude. But your sister is the one that is tripping over all that treasure and may feel you're spoiling them. She may also be distracted with the hectic Christmas season, as many parents are.
It would probably be better to base your appraisal of your relationship with your sister on the other eleven months.