My brother and I have a very similar relationship
. Actually, the warmest it's ever been between us was because of Doctor Who - I got him watching it, and since I'm the only person he knows who watches it, if he wants to comment on an episode he has to text me.
We really have little to no actual relationship
outside of being blood.
I have something of a pet theory about this, though. *puts on thinking cap* I think people - almost always other family members - get concerned when there doesn't seem to be a strong bond between siblings (or parent/child or whatever) not because they care so much about the individuals involved, but because family is such an integral part of our lives, and these days people imagine it to be much weaker than in the mythologized past. People yearn for the close-knit, loving bonds they imagine families should have and it distresses them when reality doesn't reflect that.
So the best way I've found to deflect those people who cry "faaaaaaaamily" is to reassure them on their own terms. In addition to the previous posters' suggestions, try an answer that focuses positively on family in general and kind of bean-dips away from your sister.
"Oh, you know me, I'd go to hell and back for family but Sis and I just don't get along."
"Of course, family is so important. Sis and I just don't get along, but I never forget she's my sister!"
"Oh, I am always careful to stay close with family, Sis and I just don't have a lot in common."
Or even something more vague:
"Of course family is important! Speaking of, have you heard about grandma's new boyfriend?"