I have legal guardianship of my niece. Can I claim equivalent to spouse for her

Your sibling's children do not qualify as Dependents for the Eligible Dependent credit.

From CRA web site:

In addition, at the time you met the above conditions, the dependant must also have been either:

• your parent or grandparent by blood, marriage, common-law partnership, or adoption; or
• your child, grandchild, brother, or sister, by blood, marriage, common-law partnership, or adoption and under 18 years of age or had an impairment in physical or mental functions.