Help- I think my best friend has another sister she never told me about?

[tayg]

Ok so we've known eachother for nearly 8 years and have grown incredibly close in the last few years. However, I have always been a little suspicious about her family life. For one thing I'm just about always the one to host while she RARELY reciprocates (ive been to her house a total of 3 times in 8 years, and only once did i stay overnight, while she's spent countless holiday breaks, summers,etc at my house including this one) yet she always seems to weasel out of having me over her moms house or makes an excuse..never really invites me. I have met her younger brother and sister but she rarely talks about them. Being the naturally curious person I am i decided to do some investigating and started following her brother and sister on twitter and facebook. Sure enough there's another girl that is mentioned in many of their tweets(and pics) with the same last name as my friend. I had asked my friend about this particular girl before a few years ago and my friend told me the girl was her cousin and abruptly changed the subject. I'm actually kinda in shock- i haven't said anything to her about it yet because it seems like a touchy subject and I don't know how she'd respond. This also isn't the first thing she's lied about of her background(caught her in a few tall tales she later came clean about). My guts telling me not to address it since she obviously has her reasons but cant help but think this is kinda shady especially since the girl seems perfectly normal and nice judging by her pics and profile(she was also quite pretty). At any rate, what would cause someone to lie about such a thing? Should I call her out on it? Any insight or experiences would be great to hear!
just for the record were both college students in our early 20's. her parents got divorced a few years ago but were still together when i met her, and the other girl in question is only a few years younger than her. im starting to wonder if she has a deep psychological problem as mentioned above this wouldn't be her first 'big' lie. when i first met her in HS i assumed she came from a very wealthy family due to the impression she gave off...later found out(through her own admission) her dad was unemployed and her mom worked like 3 jobs, they were in the process of divorce and she lived in a modest sized townhouse(which she was even hesitant to show me for the past few years of our friendship)

[Rachel]

Maybe the girl was outed by her family. I've heard of things like that. Or maybe the girl did something really horrible to your friend or something like that so she doesn't want to relive the past? I'd stay quiet about it. Its obvious that she doesn't want to talk about her and I don't think you want to ruin your friendship over this. Because it really is just your friends business not yours.