Well not literally but you know how a dog pees on things to mark territory, my MIL does something similar, just without the pee (as far as I know.LOL)
In my case, my MIL battle is a battle that MIL has launched and refuses to give up the fight that my husband and my BIL are her territory. I have noticed this territorialness goes beyond her sons, let me share some examples of how my MIL marks her territory.
When I first came into the picture, MIL marked her territory of my DH, her son, by giving him a never ending sonny do list. One that would take up all his time and resources and leave very little left for me. (which by the way, did not last long)
Baking and cooking: these are big ones with her. Baking and cooking is MILs territory. If you bake or cook she will sabotage. I have seen the sabotage physically and in her words, going around warning everyone not to eat my buttertarts....She caught me using my hands. If MIL is invited anywhere where another will be baking, she will insist on bringing something that she baked, if you do not eat hers she will strong arm and take it as a personal rejection even. I have been threatened by MIL for baking told to "watch out" even. I can see the level of her competition with the targetting person even in how much baking she brings. She is very competive with me so a whole large cooler full of her baking is brought to my place (I am serious) yet at SILs as she is less competitive (BIL was not her surrogate spouse, my DH was) she will bring a roast or a cake, not not a whole cooler.
Rooms in homes: At BIL and SILs MIL will actually fill her trunk with stuff like blankets, pillows, herbal concauctions she brewed up (MIL fancies herself a herbalist). MIL will literally mark the downstairs rooms of their home as if they belong to her. She will put her junk in the bedroom and retreat to it on regular intervals as it is "her room". MIL will set up the downstairs washroom with her herbal concauctions as sort of a treatment center for whatever ailment she is ficticiously claiming she has at the moment (very important as this is BILs hot button for her to play him with). She will do this so that the bathroom is unusable to everyone else.
My DD was claimed as hers. She would hold on to her with dear life when she was allowed around her (not for years now) as if DD was hers. She would answer back when my DD called for mom, she would interfere and answer questions directed to us about DD as if DD belonged to her, she role played that she was the one responsible for DD.
The most amusing marking of territory that I have seen yet, happened recently. MIL wanted a roof that she thought she could get for free from the insurance company due to a hail storm. So instead of MIL dialing the insurance company herself she calls a family meeting, that SIL and I were not part of...see MIL is marking the territory of her sons as hers to SIL and I by excluding us from the family in this manner.
Prior to the meeting, MIL walked around like a very important corporate leader with these files in hands with much gusto. Called this meeting that looked more like MIL was a manager calling her staff that was BIL and DH to make a decision. At the end, MIL the manager decided to not call the insurance company, and I watched with amusement as MIL blurted out that there is only 10 days left to make the claim as if she was the manager delegating the task to her staff.
SIL and I sat at the sand box, excluded from the meeting. MIL wanted to make sure we good and excluded by speaking German instead of English, that SIL and I are the only ones that don't speak it.
This was MIL marking her territory to SIL and I, in a very aggressive bull dog style sort of way that only someone as mentally disturbed as she is could think of, that BIL and my DH are her territory but in a very desparate and utterly ridiculous fashion, but amusing at the same time. They tend to get like this when they start to realize that they are loosing their grip on their territorial properties of their children I have noticed.
MIL thinks she is winning all these little battles, but the truth is that she is already the looser to have to act like this in the first place. It just shows how unstable and distorted she is, as well it reaks of desparation and being pathetic.