It's most probably a Chinese historical/wuxia drama.
The female lead and her sister gets separated when they were young. Sister works as an assassin. Our FL and her sister fight a few times with out knowing their relation. IIRC they fight in a secret/underground room left by their family members. And there is also a swing that they used to play when they were young. They both seem to remember the swing partially.
Please help me find this drama.
