"This is my sister."
Translation:यह मेरी बहन है।
In this question something is off with the correction. I got the error message «You missed a word. यह मेरी बहन है।» with only the danda/full-stop character underlined. But punctuation should not count at all (at least it generally isn't on duolingo).
My answer was «ये मेरी बहन है». So there is an actual error in there, that seems to have gone unnoticed: I wrote ये instead of यह.
I hope this helps finding the bug behind the odd error message. I could only report it as «The "Correct solution" is unnatural or has an error.» but that is probably not very clear. Thanks in any case to the people who made this course. Keep up the good work!
Your sentence is not correct. मेरी बहन means "my sister". Here, "my" could either be a male or a female.
Similarly, उसकी बहन could either mean "his sister" or "her sister".
Combining both उसकी and मेरी does not make sense.
Please refer to this link for concept clarity - https://www.duolingo.com/skill/hi/Introduction/tips-and-notes