Yes, Sam Smith does have siblings - he has two sisters to be precise.
I found this out because he recently posted a few Instagram photos of himself spending time with his sisters. Not just a regular day out either - they were at a tattoo parlour!
However, even though the three of them look very close - Sam admitted in a magazine interview that he's not actually that close with his siblings or the rest of his family.
Sam even declared that last Christmas, he made plans to hangout with his boyfriend instead of his family, because whenever he gets together with his family over Christmas, it usually ends in drunken arguments!
You can read all about that here if you're interested in learning more about Sam Smith and his siblings:
http://www.dailymail.co.uk/tvshowbiz/article-2886452/Sam-Smith-gets-digits-tattooed-sisters.html