<human>: So this might be long. On mobile. Names have been changed and throwaway for privacy. 

My sister, Ally, has always had the best boyfriends, she has been with mike (22-3m) for about 5 years, they have a two year old son. 

Mike is an heir to a large company that makes multi millions a year, he currently works there and clears close to 100k after taxes. Needless to say Ally has had everything she's ever wanted from mike, new cars every year, all the best clothes, they bought a 350k house the week after my nephew was born. Plastic surgery the works. Mikes family is usually pitching in plenty of money for them to also spend. 

Mike was diagnosed with cancer about two years ago, luckily he is currently in remission and is back to work and on track of being healthy. Ally was his caretaker at home and being a stay at home mom for the 9 months of chemo for mike. Shortly after he was finished with chemo a friend of mikes moved in their house, one thing led to another and ally was sleeping with the friend. Mike shortly found out about it and ally broke it off with the friend. And all was well

Well fast forward to now, ally is doing it again, she has sent me numerous snapchats of her and this new guy Andrew. Apparently they met through some friends she now works with. She has sent me snaps of them laying in bed together and throughout the day of them together while mike was away for a trip. I do not know where my nephew is during this, but I asked her what was going on and she said she is cancelling the wedding that is scheduled for next September and she is tire of Mike. Basically the same spiel that I was told with the first guy. 

Here's my problem. Mike is a pretty nice guy, and I don't want to see him get hurt, but ally is my sister. And I don't know where or what I should do with all this information

Give me the gist of the text in a nutshell
<bot>: my sister is looking to be a serial cheater. I have seen proof and she has admitted it to me. Wtf do I do? Stay silent and let this happen. Or tell Mike and risk my relationship with my sister.