Skype Mac to Mac audio problems (echoes)

Everytime when I talk with my sister,who has same iMac as mine (iMac mid 2011) I have problems with the audio,I hear echoes of my voice and it's so annoying cause I can't hear what she is saying.I have this problems only when I talk with my sister (Mac to Mac),because when I'm talking with other people who use Skype for Windows I don't have this problem. Maybe someone can help me 🙂

P.S. I tried to turn on/turn off automatically adjusting microphone settings in Skype preferences.