"His two sisters have a child."
Translation:Seine beiden Schwestern haben ein Kind.
No, it should be ok. I would use "His two sisters" = "Seine zwei Schwestern", and "Both his sisters"="Seine beiden Schwestern".
"Seine zwei Schwestern" seems to capture the denotation better than "Seine beiden Schwestern."
Are youa native speaker? My experience is it's more common to say it the way duolingo have it 'seine beiden Schwestern' although it sounds strange to us.
But does someone know what this sentence means? The daughters have one child between the two of them, or they have one each??
I'm pretty sure it could mean both. If you say "both of his sisters have a child" then you'd probably think there are two kids as well.