Sirloin meat can be used for stew but it's not the best meat for stew as it will become hard as it cooks in the stew.
If you want the best tasting and easiest to eat stew it's best to use chuck meat to make the perfect stew.
I tried using sirloin for making stew and it was okay to eat but It was a bit tougher trying to eat the sirloin in the stew and wasn't as enjoyable.
But when I use chuck meat like most people do when making stew the stew turned out and still turns out just fine like it should.
If all you have is some sirloin to use to cook in some stew you certainly can use the sirloin in your stew but if you can use the chuck meat for best results.