Is Otohime really the mother of Shirahoshi? One of the interesting and weird things I saw in One Piece was the mermen and mermaids in the Fishman Island Arc. But one of the biggest confusion I have is the relation between Shirahoshi and Otohime. I don't know how Shirahoshi can be a real daughter of Otohime. Otohime having such a small body and Shirahoshi being huge even in baby stage. Anyone has any ideas?
I found this in SBS Vol. 10

D: When the Arlong guys and the other Fishmen give birth, do they lay eggs like fish?
O: BZZZT. Fishmen are still mammals, so they give birth the way humans do. There are fishwomen too, of course.

From this we can confirm that Shirahoshi was not born from egg. Fishmen and Fishwomen are mammals.
If Shirahoshi was not born from egg the most logical explanation behind her birth is that she was probably born small and had a massive growth spurt. Growing to a massive growth is not impossible because she has royal blood and her father Neptune is of a giant size merman after all.
So the answer to your question is yes. Otohime is the mother of Shirahoshi even with differences in their size.