Translation:František is her mom's older brother.
"František is her mother's older brother" should be an alternative translation surely?
Well, máma corresponds to mom, and matka corresponds to mother. It seems to me that this lesson is focusing on the mom/dad words instead of the mother/father ones.
The sentences 'Mám maso pro mámu' and 'Její táta je mladší než její máma' accept 'mother' and 'father'. I haven't come across any other DL course that insists on differentiating between formal and informal terms for parents (e.g. Russian 'мама' always accepts 'mother'), probably in part because the informal English terms can vary considerably with location, and 'mother' itself can also be an informal term of address.
Considering that the creators have accepted 'mother' for 'máma' in more than one sentence, it's probably worth reporting in others.
Differentiation of formal and informal addressing is the rule here. This is also true in the reciprocal English course. In Czech, the difference is significant. In the examples above, it is rather an exceptional oversight. (I'm sorry for my bad English)