Good one. Honestly, I don't tend to let this stuff control me, (or at least I endeavor to not let it control me) despite how it might make me feel. Point being, if I wanted to remove her as a friend, I would do so, then explain to her why (felt I wanted Facebook to be more private) and that would be the end of it; if she or others don't understand and attack me (so, you) for that, then that's not my (your xD) problem. Of course, I wouldn't add her in the first place.

My dad's online, but not on Facebook; if he were, I can see me being tempted to have him on my Facebook, but my Facebook is private-ish; he respects that.
My thinking is that if we never stand our ground about these things, we'll constantly be hiding and walking on egg-shells; drives me crazy. Take my parents for examples, they won't tell my step-mother's parents to basically stop bringing tons of food over (really wasteful amounts) because they can't eat it all, don't like it all, and/or it just goes to waste. Personally, I think it's ridiculous, and if they'd let me, I'd just tell them myself. Problem is, they left it soooo long that now it REALLY is an issue because they've lied for ages, which is just not the best way to handle it, IMO.
So, ... I guess what I'm saying is: do you want to drag this on for ages, driving you crazy, making you feel uncomfortable, all with her being none-the-wiser,
or, do you want to stop it here and now? I'm sure they will recover and time will go on as always. Also, logically, I'd have thought that what goes on between you and your mother has nothing to do with anyone else, so, ... with that in mind, I wouldn't really care of the others decided to kick up a stink over something that has nothing to do with them, ... besides, it's just Facebook; it's not a real representation of years of friendship and family, ... or is it?
That's just me, anyway. That's just how I would do it. Is that cold, maybe? I don't know - I do things in weird ways, apparently.
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{ Kein Teufel }
Translation: Not a devil
[ `id -u` -eq 0 ] || exit 1