Sister had with my e fiance?

My sister just found out she was prego about 3 days ago.and She has been hanging with ex alot.And now he just sent me a text saying that,Its his baby and they have been having ing behind mine and my sisters fiance's back.I dont know what to do or say.I am so upset.How Could she do this to her own sister?Is There a way that she can have a Dna Test done befor she has the baby or does it have to be After the baby's born?I wanna know if he is lying or not bec. he has never lied to me the whole four years that I have known him.Thank YOu.