The sentence does not survive these
Possession the other way round
The rule
There is no genitive: possession is built from the dative plus a possessive, or with vo.
Em Peter sis Auto.
Peters Auto.
De Anna ihri Schwöschter.
Annas Schwester.
S Dach vom Huus.
Das Dach des Hauses.
Where it catches you
Em Peter sis Auto is not a mistake, it is the normal form. The person comes first, the thing after.
Where this actually comes up
The same structure, in sentences people actually say.
- Visitors
Em Herr Meier sis Zimmer isch dört äne.
Herrn Meiers Zimmer ist dort drüben.
De Anna ihri Schwöschter chunt au no.
Annas Schwester kommt auch noch.