I gave two possibilities, you choose or they maybe both true.
It could be that your mother sees your sister as less independent and more in need of her help. You, on the other hand, are perfectly ok, you don't need as much time and attention as your sister and she has learned not to look too much after you.
You also wrote that your sister's kids need your Mom and she likes that. So your Mom is a person that needs to be needed and glories in this feeling.
possibly you have always acted the over-independent and your sister the over-dependent.
What do you really want from your Mom? attention for your kids or for you? ask her to tell you that she loves you. Write her a letter, you hardly ever forget a letter! and express your real feelings to her. What do you want from her?
Could it be that you are reproaching her all the times where you were in need but you forced yourself not to express your needs? Maybe it's time to just express them, without using the scale:me more, you less.
Tell her what you really want from her.
But remember that even Moms have flaws, so if she needs to be needed and you need to be independent.... it's hard that you meet in the middle.