Entry into and residence in Latvia for citizens of the European Union, the European Economic Area and Switzerland and their family members, is provided for in the Cabinet Regulation of 30 August 2011, No. 675 „Procedures for the Entry and Residence in the Republic of Latvia of Citizens of the Union and their Family Members”. For further information see here.
To read about visa facilitation that applies to family members of the citizens of the European Union, the European Economic Area and Switzerland, see the information here.