From an essential oil point of view:

Peppermint has emmenagogue properties, which is probably why it's not considered safe to use during pregnancy. Some aromatherapists consider it safe to use topically (i.e. on the skin) during pregnancy, but not orally, but others will avoid it during pregnancy altogether to be safe. I've been avoiding it myself (which is a PITA, cause I normally use heaps of peppermint for headaches/migraines and to prevent vomiting, which would have been perfect in my first trimester).

Sage is also generally considered unsafe during pregnancy, because it can have an influence on your oestrogen production and can cause uterine contractions.

Sorry to be the bearer of bad news .... it sounds like it was a lovely thought from your Mum ...

If you're looking for some good essential oils for swollen feet, I've been using lime, tangerine, sweet orange and grapefruit, which have diuretic properties (they're also all relatively cheap essential oils lol).