Can I marry my son's mother-in -law

There's nothing legally against it. However, your son, or his wife, might not be pleased if you do so. Have you talked to them about it?

I am confused and think you are too.

What does she say about the idea?

What does she say about the idea?

She is a widow and I'm a widower both in are 70's

What about the rest of the family? How do they feel?

This is your sons, wife's mother.

Nothing illegal. And of course you would already be dating if you are talking about marriage. Have you discussed this with your son or her with her daughter

What about the rest of the family? How do they feel?

They are totally unaware. They know we communicate all the time by Email and phone. We are 3000 miles apart

Then the first thing you need to do is talk to them about it. You don't want to alienate them by keeping it a secret.