"Do you have relatives there?"

Translation: Har du slektninger der?

[Poker87]

Why is it not allow to say 'dit' instead of 'der'?

[Luke_5.1991]

"Dit" suggestions motion of some kind.

[...marcus]

It's like "here" and "hither",
or "there" and "thither" in english.

"der" describes something stationary, and "dit" something in motion.