And the issue of Turkish support for Islamist terrorist gangs predates ISIS' entrance into the matter.
When we're finished looking at Reyhanli, let's go one border crossing to the west, to
Kasab near the med coast, where the Armenian Christians live(d) (Village with several churches to find just north-west from the border crossing). In March 2014 the Turks decided, again after a similar yet much bloodier event in 2013, to let a bunch of their favorite goons storm over from their side of the border into Syria and
have their way with the population and their holy sites.
On the talk page of the wiki article I saved the following video with the remark "for those doubting that these things happen", so why not show it off.
I just see that embedding is disabled for it, so
watch it on YouTube.
There's no violence or anything bloody in it, just a 4 min visit at the border station you've just seen from above, per annotated "rebel" videos. I don't know the logo to the right, but top left are Harakat Sham al-Islam, Islamic Front and Nusra Front (aka Al-Qaeda in Syria). Hardcore Islamists, with them a gang of vicious Chechens calling themselves Junud Al-Sham. If I'm not mistaken their red-bearded leader was just recently killed in an airstrike.
This is going on since the earliest days of the "civil war" under the eyes of and with active contribution from Turkey.