I'll petition an I-130 for my mother after I become a citizen in early 2007.
Since her original birth certificate is full of errors, I have been told
she will need an affidavit from one of her siblings(her parents not alive)
proving her date of birth.

My question is; If she obtains an affidavit from her sibling now, will it be good a year and a half from now, when immigration will actually ask for it?

I am asking this because her only living sibling is getting sicker and may not last
into 2007.

Any other suggestions?

YES you cld make the affidavit and notarize it immediately......when the 50-60 and more years old piece of document is valid then why will this affidavit will become invalid..........thats why we have to NOTARIZE it........

the more annoying thing is about the validity of PCC........you know just compare to the fact that the
police clearance certificate is valid till six months time.....just telll me Is their any guarantee that a person will not commit any crime after getting police clearance certificate.......i mean between the time of receipt of PCC and interview anything can happen.......but the PCC is valid for six months.......so dear this affidavit is valid for the whole life...........hope this helps you