A crack at the bridge will not affect tuning stability but will cause
false beats from allowing the pin to vibrate, a vibrato effect which
is most annoying.

CA glue can be applied cautiously while the piano is on its back.
Thin viscosity will seep in and do the trick, and a medium could be
used following to fill the gap if you have the ambition, although it
is not necessary.

If the cracks appear to have been there for a long time and have
not become worse, then chances are they are stabilized already.  So
arresting the cracks may only produce a more solid tone from a more
rigid pin while tuning stability is still at the mercy of atmospheric
changes, general pin tightness and tuning hammer technique.

Regards,
Jon Page,   piano technician
Harwich Port, Cape Cod, Mass.