what do I do?

So.... My sister and her boyfriend came into town so we could all go and take my aunt out to eat for her birthday. When we were done with that we came back to the house and all chatted awhile. Then it was time for them to leave to be at the house when my niece got out of school. My sisters boyfriend (we will call tom) was waiting for some papers to be sent to his phone and didn't want to take it because it was dying and needed to be charged plus he needed the wifi to get the papers so he left his phone and said they will get it on Wednesday when they come back into town. ( this all happened Monday). Anyway, later that day my sister called and gave me the code to the phone with him next to her so I could check and see if he got the papers. They said to click on Whatsapp. So I did and it said new messages and clicked on the first one, nothing but a bunch of pictures of a woman and him. So I clicked the next one same thing. Then the next one still a woman but a picture of some papers so I figured that was it and said yeah it's here. They said thanks and hung up. But me, being nosey, went back to see what the messages with the women were about. Well of course it wasn't in English. So I didn't want to screen shot and send to myself because that phone was an android and I can't work those so I just took my phone and took a picture of all the messages and then sent them to my friend and asked her if she could translate for me. She did and said that he was asking the lady to come to the U.S and they could buy a house and be together. He can't live without her and that she's so beautiful they should be together. At first the lady was into it then said no, that she didn't want anything to do with him, because he was a rebel and she didn't like that.  He said he just does what his heart tells him and follows the Lord. I was pissed!!!!! My sister is pregnant with his kid, due in Oct. he was sending these messages in July of this year. I don't know how to tell her. Not to Mention he announced to the whole family one night a few months ago while my sister was in the bathroom that had she not been pregnant he wouldn't be with her. WTF?!!! I feel so bad for her.