I'm personally kind of weird about using designs that represent a certain heritage if you're not of that heritage (like kanji, celtic knotwork, tribals, etc)... But that's just me.
I can tell you, though, being of Celtic heritage myself (I'm of Scottish and Welsh descent), I've done a lot of research on Celtic knotwork and there just isn't any real consensus on what the knots mean. There is a school of thought that while the basic idea behind the knotwork is continuity, and unbroken line, that knotwork itself doesn't really mean anything, its just decorative.
If I were you, I'd start looking for something that in your mind and in your heart reminds you of your relationship with your mother. It could be anything... If I were close enough to my mother to have a tattoo done with her, I have several things that immediately bring her to my mind... Yellow roses, zinnias, sandalwood fans, and oddly enough, the Grateful Dead head. One of my best girlfriends, her sister, and their mom all had tattoos done at the same time... They picked the flowers that represent their birth months (each month has a different flower), and they each had the two flowers that represented the other two tattooed. It was a really sweet way to get custom tattoos that weren't quite matching, but that were a piece of each of them.
Talk to your mom. See what she says when you ask her about things that remind her of you. Or talk about memories that you both share of things that you've done together. This could be a real opportunity to work together to have something amazingly special tattooed.
