Trying to iMessage one person, sends to a group

Hello,

I have been trying to iMessage my sister, but lately when I try to do so, my conversation with another person pops up, as well as my conversation with my sister, which seem to be combined in a group message. I never created this group chat. I cannot message my sister individually without sending it in the group. I tried the basic things- restarting my phone, turning off and on iMessage. Any help is appreciated!

iPhone 6, iOS 8