that was just my question, I'm still lost with the different cases. Doesn't anyone know a trick to make the difference, or is it just time and practising that will help?
"They are selling Martha the house" and "They are selling the house to Martha" both convey that Martha is the indirect object. However, the excercise currently only accepts the second form.
Only: Sprzedają Marcie dom - makes sense.
Sprzedają Martę means - they are selling Martha (to someone)
What case is Marcie in, please? And I can't help but think that it wouldn't hurt to have a preposition between dom and Marcie.
Actually I don't see anything at all in "Ona jest mamą" that would suggest she's my mom. There isn't anything about 'me' in the sentence. Compare with "Rozmawiam z mamą" (I am talking with [my/] mom) - here 'I' am present in the sentence, so it's most probable that I'm talking with my own mom. Generally this 'presumption of ownership' states that the object is probably 'owned' by the subject of the sentence, and your sentence has "She" as the subject.
As for "their house", that actually makes a lot of sense. Added.