${\mathrm{MgBr}}_2$ and ${\mathrm{MgI}}_2$ are soluble in acetone because of:

${\mathrm{MgI}}_2$ and ${\mathrm{MgBr}}_2$ are ionic compounds.

Because Br and I are larger and have a higher solvation energy, these compounds are soluble in acetone but insoluble in water.

So ${\mathrm{MgI}}_2$ and ${\mathrm{MgBr}}_2$ are soluble in acetone due to their covalent nature