Iphone messages only working one way?

Is there some setting you have to change to make it either texts or an iMessage?

My sister just got an iphone. When she sends me things it sends as an iMessage, but when I text her it's an actual text. So I figured it was probably something on my end, but I text other people with iPhones and don't have this peoblem

I have this problem with both of my parents when i text them. We've found that sometimes it works, but sometimes it doesn't. You can go to your settings and turn the 'Send as SMS when imessage is unavailable" off. It worked for us.