Badger´s mate is right: the hotter the variety of chile, the hotter the sauce, in principle.
The hottest chile in the world right now is the Carolina Reaper, at about 2.2 million Scoville units.
Next comes the Trinidad Scorpion, at around 1.4 million, with the Naga and Bhut Jolokia not far behind, at about 1.2 million.
the hottest habanero, the Red Savina, is a lightweight at 600,000 SU.
A tiny weeny piece of the Reaper, about the size of a pinhead, will still be burning 30 minutes after consumption. It is insanely hot.
I´ve had hot sauces containing the -jolokia chiles and they´re really good. There´s a Mexican sauce which i absolutely adore ( and recommend) called Chimay. Their extra-hot is brilliant.
So look for hot sauces with the above in them and you´ll probably be able to send your SIL on the Highway to Hell
Avoid all the traditional sauces - Tabasco, Cholula, Sriracha, etc. They´re not going to cut the mustard with a dedicated chilehead.
One thing that WILL make a sauce hotter than ever is if it has oil in it. Since the capsaicin is soluble in oil, it intensifies the flavour of the sauce.