"Mater, matris" is a third declension noun and "severus, severa, severum" is a first and second declension noun. Thus, their endings may look different, though they will agree in case (here: accusative), number (here: singular) and gender (here: feminine).
My remark is not particularly about this sentence, but I have reported it because of a wrong audio. Each time there is a form of habeo a certain female speaker (the one who read this sentence) pronounces a long a, but it is short.
Habes, habemus, and habetis all have a long E, pronounced "ay." Habeo, habet, and habent should have the short e as you mention.
I was talking about the a in the first syllable, which is short in all forms.
Long and short “a” both sound like the “a”in “ahh”. It’s just that long “a” is held slightly longer.
That's not what the sentence says. While the same (or similar at least) information is conveyed, if one's goal is to learn a language, best not to stray too far from given vocabulary.