We get our DNA 50%/50% from our biological parents. Back any further than that, and it's all statistics. You get on average 25% of your DNA from each of your four grandparents, but that's only an average. Your parents pass along half of their mix of their two parents, but your bundle might contain more of, say, your maternal grandfather's contribution to your father, where your sibling's bundle got more of your maternal grandmother's contribution to him. Same with your mother.
A helpful way to visualize this is to picture your parents as having a bag of 100 marbles each. Mom's bag has a 50/50 mix of light pink and light blue marbles. Dad's is a 50/50 mix of dark pink and dark blue marbles. Each of them reaches into their bags and pulls out a random assortment of 50 marbles to make you. (a sorceror then replaces the exact ones removed, to make room for sibling to come).
20 lt pink, 30 lt blue; 24 dk pink, & 26 dk blue
When mom & dad decide to bring you a sibling, she gets
29 lt pink, 21 lt blue; 27 dk pink, & 23 dk blue
You each got 100 marbles, 50 from each of your parents, but you got different mixes.
Down the road, you're passing along more of your maternal grandfather. Your sister's descendants have more of your maternal grandmother to pass along. If your Native American ancestor is represented in those light blue marbles of your maternal grandfather, you got a lot more of it than your sister did, and your kids are more likely to have that signal persist than hers are.
Of course, the marbles are of an overly simplified model that doesn't take into account the lengths of the unbroken chunks of DNA. When they get very short, it becomes difficult to attribute them to any one person. A good example involves me and my full sister. She matches a LOT more people than I do, especially on our father's side. She didn't get more of our father's DNA - remember, parental DNA is the only guaranteed 50/50 contribution. But she got longer unbroken pieces of it. In general, my dna got chopped up a lot more on both sides.
By the time you get more than 3 generations out, you start to lose connection to a lot of cousins. Heck, my sister has 1.6x more in common with our 1C1R than I do. Imagine the difference even one more generation down, let alone 5, 6, or more.
Your 2% isn't a whole lot, but that combined with your alleged Native ancestor suggests to my amateur eyes that it may be real. A number of tribes, such as the Seminole, made common cause with many African Americans. In other cases, it was more socially acceptable to pass a darker skin tone off as an "Indian princess" ancestor than an African one, and that story gets passed down the line. Do you have other near relatives to test?
A terrific article on the significance of small amounts of DNA, and how limited ancestry of a single ethnic group can wash out in only a few generations' time is here: http://www.rootsandrecombinantdna.com/2015/03/native-american-dna-is-just-not-that.html