Is Queen Victoria related to Queen Isabella?

Very very indirectly. They are both descendants of John of Gaunt, so they are descent cousins.

John of Gaunt+

1.Constance of Castile

Katherine of Lancaster-Henry III of Castile

John II of Castile-Isabel of Portugal

Isabella of Castile

2. Blanche of Lancaster

Philippa of Lancaster-John I of Portugal

Infante Joao of Portugal-Isabel of Bragnanza

Isabelle of Portugal-John II of Castile

3. Katherine Swynford (mistress of John of Gaunt whom he married in 1396)

John Beaufort-Margaret Holland

John Beaufort-Margaret Beauchamp

Margaret Beaufort-Edmund Tudor

Henry (VII) Tudor-Elizabeth of York