am i overreacting..

me and my sister in law are very close and tell eachother everything but lately she has been cold towards me so i asked what was wrong have i done something and she said no and i was like theres something on your mind tell me and she told me she is talking/seeing my uncle i got really angry and just walked away because thats my uncle my sister in law and my aunty just doesnt seem right.. oh and my mum is my hubbys sister in law am i over reacting???