What I find less common- not impossible, but less common- is that this is something she is only now beginning to do. I, like you, know of many women (and yes, mothers) who travel alone for any number of reasons.
In my experience (and please- I am not saying your experience is less valid than mine is) most of the women who are married with young children and regularly travel abroad alone are women who have done so their entire adult lives. They were jetsetters from the beginning, and didn't stop being that person simply because they got married and had children. Or, they'd always wanted to be that person, and had a financial windfall- huge promotion at work, came into an inheritance- something that altered their financial situation substantially enough to make these trips possible without massive strain on the family budget.
I also know several women who began travelling- again, alone- for the first time after their children were grown and out of the house. I know several women who have no children who began travelling along after a divorce- rediscovering themselves, that sort of thing.
In MY OWN EXPERIENCE (mine- just mine), however, I don't know anyone who fits the scenario we have with Sarai. She has never been outside of the US, she is not financially well off, and she has small children (I don't even care that she was married- that's the least of the weirdness for me).
It is unique. Actually, it's a shame that is so rare- but no, it is not common- to suddenly develop the hobby of world traveler under those circumstances.
I'm not saying there's anything wrong with the fact that she DID travel outside of the US. I don't think there's a thing wrong with it. In fact, what's wrong is that it isn't more acceptable, but regardless, in THESE circumstances, it isn't as common a thing to do.