this is /apni/ whereas you are thinking of /aap-ki/. See the difference? apna/apne/apni means "one's own/ self's". There's a grammar rule that says one must use this word instead of my/your/his/her/our if the thing is being possessed by the subject of the sentence. You can say "I am with his mom" or "He is with my mom", but you can't say "he is with his mom" or "I am with my mom" :) You gotta say, "I am with one's own [aapni] mom" or "He is with one's own mom".
To all fellow learners: I do not blame Duolingo for "my own lack" of intuition (oblique), but please don't consider this a critique of "your own lack" of intuition (not oblique).
[NB: true, honest statement, so I hope this helps, as it did help me. (I also hope it is the correct understanding of this grammatical point !]
hari om tatsat
Yes. There is no ambiguity in this case but it sounds weird.
The difference is more pronounced in other cases. For example, both नेहा अपनी माँ के साथ है and नेहा उसकी माँ के साथ है translate to 'Neha is with her mother' but convey different meanings. The first sentence is saying that Neha is with Neha's own mother and the second that she is with someone else's mother.
अपना/अपनी always refers to the subject of the sentence (or the subject of the clause that अपना/अपनी is in if the sentence has multiple clauses). Since the subject of the given sentence is 'I', अपनी translates to 'my'.
If the sentence was 'वह अपनी माँ के साथ है' instead, then the subject of the sentence is वह(he/she). So अपनी is 'him/her' with the sentence translating to 'She is with her (own) mother'.
If you want to write 'I am with her mother', you have to use the actual word for his/her (उसकी ). 'मैं उसकी माँ के साथ हूँ'.
के means OF
साथ means WITH
But the common phrase to express "with" is के साथ.
A rough analogy*: "I got OUT OF the car." You could say "I got out the car", but it's not really as complete sounding as "out of the car." Point is, "out of" goes together as a phrase.
*This is not a grammatical comparison, just an illustration of 2-word prepositional phrase in English!