No particular reason, just a coincidence, at least if you look at the etymologies:
Son ← Old English
sunu ← Proto-Germanic
*sunuz ← Proto-Indo-European
*sunu-/
*sunyu-, from root
*su- "to give birth"
Father ← OE
fæder ← PG
*fader ← PIE
*p@ter, presumably from baby-speak sound like
pa
Mother ← OE
modor ← PG
*mothær ← PIE
*mater-
Daughter ← OE
dohtor ← PG
*dochter, earlier
*dhukter ← PIE
*dhugheter
Source:
http://www.etymonline.com/