Personally, I used to be much more selfish when I was younger than I am today. So I would argue based on my own experience that no, you do not get more selfish with age.
As for your situation, from seeing my mother and how she acts, if they are anything alike, it is not so much to do with her biological family, as it is to do with not being able to be bothered to put in any more effort than she already has to.
Last week my father organized a social event, dinner at a restaurant for 30 people. A couple days before the dinner, he was informed that an additional person would be attending, and he made a big drama out of it, claiming how he had to redo all the seating arrangements. He made a big deal, out of printing a name on a bit of paper and telling the restaurant manager he would require another chair, to such an extent, that you would think someone had just told him he would be in charge of logistics for the next Olympics.
I would imagine, your mother was probably thinking “well fuck me, that’s another trip to the grocery store and another hour in the kitchen, thanks” rather than anything to do with blood lines or what not. She probably feels she has the full work load, and is just tired of being the one who always has to do everything for every Christmas, thanks giving, Easter, and so on.