Is it permissible to take a portion of the building materials belonging to my indebted brother, which he stored in my warehouse, and count their value towards the debt he has not repaid since 2005, even though he is demanding their return?

If you lend money to your brother and he does not return it due to his insolvency, he must be given respite. However, he must work to earn and repay his debt. If your brother is negligent in earning and has not repaid the debt, and you are able to take some of your right from iron he owns, then it appears permissible to do so, based on the opinion of those who permit the issue of seizing one's due.