Dear Abby: Fending off Facebook request
Dear Abby: My sister-in-law is demanding to know why I won’t accept her friend request on Facebook. Personally, I don’t consider her a friend and prefer not to allow her access to my Facebook page. How can I politely and honestly answer her questioning?
—Prefer to Decline
Dear Decline: Be forthright and answer her question by telling her that while she may be your sister-in-law, you do not feel personally close enough to her to be comfortable having her review your activities on a daily basis.
—Prefer to Decline
Dear Decline: Be forthright and answer her question by telling her that while she may be your sister-in-law, you do not feel personally close enough to her to be comfortable having her review your activities on a daily basis.

no subject
That sounds like Abby is one of those people who doesn't get why someone would ever want to use Facebook at all... which makes me think she's not awfully qualified to give advice in this situation!
I think people who are selective about who they friend on Facebook usually have their own criteria about it that's more specific than "I don't consider her/him a friend." If I were Abby, I'd ask what Prefer to Decline's criteria really are. Does s/he use Facebook to keep in touch with schoolmates, or for discussion of politics or pop culture, or to play games? Maybe s/he can say to the sister-in-law, "Sorry, I use my Facebook page for professional networking only, so I don't make any posts about my family, and since the majority of people who see my feed are clients and colleagues, it's not appropriate for me to communicate with family members that way."
Or just, "Facebook isn't very important to me, so I don't use it very often."
no subject
no subject
(Plus, the letter-writer should think about how much of what happens on her FB may get reported to her sister-in-law anyway. I mean, if she's friended her sibling who is presumably married to the sister-in-law, not friending the sister-in-law is pretty useless. And it sounds as though she does use her FB page for communicating with friends, and the s-i-l knows it. Which doesn't mean she should give up and friend her, but she needs to bear it in mind for her own protection.)
I think my advice would be to adapt Miss Manners' advice and keep repeating, "It just doesn't feel right to me." The answer to "why not?" is "It just doesn't" and a change of subject.
no subject
And Dear Abby completely ignores that in her answer.
no subject