How to obtain its upper level from "A style"?

Is it possible to directly obtain the same level 'B' path from myStyleA

Needs to be .parent.objectStyles.itemByName(“B”)—not .parent.itemByName(“B”). You are trying to get a object style not an object style group (which is what parent is returning in this case). 

Also you probably should make sure “B” is valid:

var g = app.documents[0].objectStyleGroups.itemByName("MyGroup").objectStyles.itemByName("A")
var i = g.parent.objectStyles.itemByName("B");

//check if the targeted object style is valid
if (i.isValid) {
	alert("An Object Style Named " + i.name + " is valid")
} else {
    alert("Style B is not valid")
}