I am married and my husband and I are looking to purchase a house. If he passes can my step child take half of our assets if we have a will?

You can leave whatever you want to whomever you want in a Will.  If your husband's Will leaves everything to you and he dies first, your Will governs.  If you die first, your property goes to the people you name.  It then becomes their property and goes to whomever you name.  Heirship and intestacy rules only apply if there is no Will.