A sire CAN be blood bound to their childe
In the 20th anniversary edition of the VtM rules there is no mention of Sires being immune to their childes blood bonds.
There is only two notable quotes regarding this in the text:
First Drink (Level one blood bond) (p287)
...All childer have this
level of bond toward their sires, for the Embrace itself forces one
drink upon the childer; they may love their “parents,” hate them, or
both, but are rarely indifferent toward them.
And on the subject of multiple and mutual blood bonds (emphasis mine)
A vampire can experience lesser (one- and two-drink) bonds toward
several individuals; indeed, many Kindred enjoy such bonds, as they
create artificial passion in their dead hearts. Upon the formation of
a full blood bond, though, all lesser sensations are wiped away.
Vampire lovers occasionally enter into mutual blood bonds with each
other; this is the closest thing the undead can feel to true love.
Even this sensation can turn to disgust or hate over the centuries,
though, and in any event few Kindred trust each other enough to
Therefore: There is no restriction on blood bonds other than once a character is bound with a level 3 blood bond they cannot have any others active.
I have checked a copy of the Revised (1998) edition; and the text is basically the same for this in the bloodbond section (p218) No mention of immunity for sires to bloodbonds.
That all said I do remember from playing LARP that there was a rule (in our group, old UK By Night) that Sires were immune to their childes blood bond; but as written in the 20th Edition book there's no immunity that I can find after repeated searches through the document. I suspect this may be nothing more than a common house rule. It would also be an excellent rumour for sires to spread to the childe ;)