It's not really that Swahili and Arabic are similar as languages, only that the millenium of trade has yielded a great deal of overlapping vocabulary.  Swahili is not a Semitic language in structure but has a great deal of vocabulary from Arabic.  Sort of like English is not a "Latin" or "Romance" language but half the words in any U.S. newspaper article are Latin.

For example, Michael is an English name.  It is also a German name, a Hebrew name, and with slight adjustments for orthography a name in every European language.  English and Hebrew are not similar langauges, however.

Maybe a better example is the name "George", which hundreds of thousands of Greek men and millions of American men including three U.S. Presidents have.  Greek and English are not similar languages unless your point of reference is 5,000 years of Indo-European linguistic development.

I mention this not to be a pedantic asshole but because Obama's heritage is in play whether it's fair or not, so may as well be more accurate than the wingers.