My contacts on my iPhone keep getting randomly deleted

I was looking through my texts and I noticed that all my contacts I was texting had their names removed and it was just the phone numbers so I started getting freaked out! Then I looked in my contacts and I still had almost all of mine but for some reason in addition to those I had my sisters contacts from her iPhone too! It's the same for her she has all of mine too for some weird reason. Can somebody please help me by telling me how this happened and how to fix it so we can get our own contacts back!

Is the iPhone 4 8GB on Virgin Mobile a good deal?

Their site is telling me I can get 300 anytime minutes per month, with unlimited access to data and messaging.

This seems too good to be true, does it not? I know you need to pay for the phone itself, but what's the catch? Does Virgin Mobile's coverage suck or something? Haha.

Added (1). For $35 per month they say

New iTunes 11 won't sync iPhone and iPad properly?

Ever since I updated to the new iTunes 11 a couple days ago. The apps don't sync properly within the two devices. It used to be that whenever I deleted an app from itunes it would automatically erase it from both of my devices. Now even if I delete a few apps they will still be on them and I have to go in them and manually delete them, its not that big of a deal but its a huge pet peeve.

How to undo allowing people to see if you read their text on iphone 4s?

I foolishly clicked yes when I downloaded the new update for the iphone and it asked me if i wanted to let people see if I read their texts. This has become an issue because I will read a text and not be able to respond right away in turn getting backlash from the person I'm in a conversation with.

Forgot 4 digit password on iPhone?

My sister forgot her 4digit passcode. I finally got it out of recovery mode and the time it tells you to wait has expired. Now it asks for the normal passcode, what can I do to get in and turn the passcode off?

Why apple doesn't produce enough Iphone 5?

Currently, Iphone 5 is not on stock.
Why do they produce so few of them? Do they want to keep people in demand?
I feel that they want to keep people in demand and with this way, people will think that it is an awesome product. I don't like their strategy

Locked out of iPhone 4s, never synced to iTunes?

My sister changed her passcode and then forgot it and now she is locked out. It's too late to get to an apple store and nothing seems to be working. I'm about to take it out of recovery mode I don't know what else to do. She's never backed up her stuff on iTunes and she afraid of losing her stuff. Please please help