Care to expand? If you are saying they both have DNA, thus they are both related, I could go for that. However, if you are saying their DNA is similar, I'm gonna have to require a scosche more evidence than a condescending sentence in passing.
In Christ, GB
The Hippo is the closest living relative, though other even-toed ungulates (including cows) are also close.
From:
A complete phylogeny of the whales,
dolphins and even-toed hoofed
mammals (Cetartiodactyla)
Samantha A. Price1*, Olaf R. P. Bininda-Emonds2 and John L. Gittleman, Biol. Rev. (2005), 80, pp. 445–473
"We present the first phylogeny to include all 290
extant species of artiodactyls and cetaceans recognised
in Wilson and Reeder (1993). The supertree topology is
fully resolved at the family level and is highly congruent
(72–79%) with the largest total-evidence cetartiodactyl tree.
It supports the current consensus that Cetacea are nested
within Artiodactyla as sister taxa to Hippopotamidae rather
than to Artiodactyla as a whole and that the sub-order
Ruminantia is a valid monophyletic clade. The other relationships
within the tree are more controversial because
no consensus exists within the literature and support is often
inconclusive due to inadequate taxon sampling."
http://blackbear.ecology.uga.edu/gittleman/pdfs/Priceetal2005.pdf
Also:
Hippopotamus and whale phylogeny
Jonathan H. Geisler1 & Jessica M. Theodor2
Top of pageAbstractArising from: J. G. M. Thewissen, L. N. Cooper, M. T. Clementz, S. Bajpai & B. N. Tiwari Nature 450, 1190–1194 (2007); Thewissen et al. reply
Thewissen et al.1 describe new fossils from India that apparently support a phylogeny that places Cetacea (that is, whales, dolphins, porpoises) as the sister group to the extinct family Raoellidae, and Hippopotamidae as more closely related to pigs and peccaries (that is, Suina) than to cetaceans. However, our reanalysis of a modified version of the data set they used2 differs in retaining molecular characters and demonstrates that Hippopotamidae is the closest extant family to Cetacea and that raoellids are the closest extinct group, consistent with previous phylogenetic studies2, 3. This topology supports the view that the aquatic adaptations in hippopotamids and cetaceans are inherited from their common ancestor4.
http://www.nature.com/nature/journal/v458/n7236/full/nature07776.html