I'd expect something with a 直してもらうよう or something for that. I'm like 99% sure the joke is she's saying a weird and awkward thing because she's a slime tbh famalam
This is a really common misconception. In most cultures, even in the past, people didn't marry and have kids until their late teens or twenties.
If we're talking specifically about medieval europe, the nobility might get betrothed at a young age, but they still typically waited before cohabiting and consummating the marriage. Commoners typically waited until their twenties, or sometimes even later -- with economic factors being a bigger concern (A: Her dowry, and B: his profession/ability to provide) and their ability to establish an independent household.
That said, the Catholic Church had set legal minimums for marriage at 12 years old for girls and 14 for boys... it just wasn't very commonly done.
The princess is making it very difficult to like her, but I can empathise with her sadness.
We haven't see a queen anywhere, so Grete might as well be her mom. Imagine hearing that from your mother, that the new kid you don't like is so much better than you, and that she'd prefer to have that kid over you.
Yes, Grete didn't actually say that, but it could be perceived that way. And she doesn't seem very sorry at all. She really doesn't like the princess that much, does she?