seguemarran Asks:

How come that the boundaries of the old kingdoms and family lands in Westeros seem to be so unchangeable? Has there ever been something like a dynastic union in order to enlarge family dominions? If, for instance (not interested in this particular speculation), Sansa came to be the only heir to the North, the Riverlands and then married Robin Arryn, wouldn’t that make a huge kingdom then united by her children? Thank you!

I don’t think it’s accurate that the boundaries are unchangeable – the histories of the various kingdoms from WOIAF show the opposite, with the borders of the various kingdoms flowing hither and yon repeatedly throughout history. In addition to the gradual and often painful process of the unification of each of the Seven Kingdoms from the hundreds of petty kingdoms that once existed, we know of a good deal of back-and-forth. The Durrandon conquest of the Crownlands and the Riverlands, the Ironborn’s conquest of much of the west coast of Westeros and the Hoare King’s conquest of the Riverlands, Gyles III Gardener who conquered most of the Stormlands before falling prey to tall poppy syndrome, the North’s attempts to conquer the Sisters and the Fingers, etc. 

As to your question about dynastic union, there’s plenty of internal examples: the Stark’s dynastic marriages to the Barrow Kings, Garlan II Gardener marrying into the Hightowers and gaining Oldtown for the Reach, the dynastic union that allowed Meryn III to successfully incorporate the Arbor into the Reach, and so on. 

However, there is a Westerosi custom that says that one man shouldn’t control more than one great seat of power, both out of respect for the balance of power, and the feudal limitations of trying to administer geographically separate fiefdoms. That likely has limited the reach and extent of dynastic marriages.