Can Walgreen's DNA testing work for siblings?

[Diane]

I'm 21 years old, and there is a VERY strong possibility that my best friend I have grown up with and known my whole life could be my sister. What I want to know is, could we get one of those DNA tests from Walgreens and see if we are related, or does that only work with paternity situations? My mother is deceased, and my friend's father, who my mother was with at the time I was conceived, lives across the country, so we couldn't ask him to participate, and my dad is not mentally well and I don't want him to know about this because it would just devastate him, I want to find out for myself. She has 5 other siblings, 2 of them are fully biological to her, the other 3 are her half siblings, and she and I look more alike than any of them. And I didn't find this out until after she was gone, but my mother terminated a few pregnancies from that man, which makes me believe even more that it can be true. If someone could help me it would be greatly appreciated, I'm going to see my friend tomorrow.