Is for fitting 970 sli plus wireless card impossible on an matx board?
It'd be very difficult. If you have a motherboard like this one which has all PCI-E slots, then it might be possible to put a GTX 970 in slot one and a second GTX 970 in slot 4, which leaves slot 3 available to install a wifi PCI-E card like this:
[] GTX 970 with ]
[]twin slot cooler]
[]Wifi card]
[] GTX 970 with ]
[]twin slot cooler]
Of course, as ricki42 points out, the mATX case needs to be able to accommodate 5 expansion slots. For example, here are photos of twin graphics cards in a Corsair 350D mATX case with a gap in between to allow another card to be installed in between the two graphics card.
Keep in mind, you could also use a USB wifi adaptor instead.