Hi, I'd like to help you with your question.
From what you said, your daughter in law has put you and your family in a very bad position. Cheating not only involves the married partners, but also the children and the extended family.
You have a couple of choices. You can say nothing and let your son and daughter in law work it out themselves. It sounds like there are definitely issues between them, and I have the feeling that her texting right in front of you both is a way of saying how angry she is about something. Your son may very well be aware of what is happening and in that case, they need to work on it themselves. If you mention how you feel, that makes for more conflict between them because they will take sides, for or against you.
You could also approach your daughter in law yourself, privately. You need to stay neutral and not be angry or accusing when you do this. Mention you are aware of what is going on between her and your son, and offer support. This may be difficult to do because it is natural for you to take your son's side, but as they say, there is always two sides to everything. Many mothers believe their kids are the hurt party, but you really are not sure so stay neutral. This action may cause ripples, but it does address how you feel about the situation and lets her know that she can't manipulate the problem and "play" other people into it.
You could also approach your son. That will appear that you support him in the situation, but if you approach him the same way you'd approach your daughter in law, very neutral, then it would let him know you are there but not going to interfere by taking sides.
Either way, it would be very beneficial if your son and daughter in law sought out counseling. If you chose to talk with them, suggest it. Even if they could do so for the children, it may be a surprise to them how helpful counseling could be. They could also speak with a pastor, if they attend church. Pastors are often very experienced in marital issues.
I hope this answer has helped you,