I wanted to ask if somebody says out of frustration, “why did Allah create relatives”, what's the ruling on the sentence?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Just uttering this expression is neither Kufr (disbelief) nor apostasy.

It may be just a question in order to know, and this is permissible. It may also be said as a way of resentment and discontent, while recognizing the extreme Wisdom of Allah, and this is forbidden.

Rather, this phrase is considered Kufr if it is said as a way of objection and aspersion to the Wisdom of Allah – and it is unlikely that a Muslim would say this.

Allah knows best.