What can be substituted for sweet fennel essential oil?

It depends on what you were planning to use the essential oil for. I will list some of its therapeutic properties with other essential oil replacements below:

Diuretic:Chamomile, Cedarwood, & Juniper

Menstrual Regulation: Juniper, Clary Sage, & Rosemary

Digestive Aid: Chamomile, Bergamot, Peppermint