English Translation
That’s my sole concern.
Does it make sense to say “Es mi única hermana”? That was also a choice.
English Translation
That’s my sole concern.
For “she is my only sister” it should be fine, but that’s not my concern 
That’s my sole concern.
Does it make sense to say “Es mi única hermana”? That was also a choice.
That’s my sole concern.
For “she is my only sister” it should be fine, but that’s not my concern