The devil is in the detail but that looks like an arrangement involving the first sibling gifting a grand a month to the spender. What would you think yourself??
Pretty simple really; Sibling A is gifting €12,000 a year to Sibling B.
The first €3,000 is ignored each year, then €9,000 falls off the Group B threshold of circa €33,500 until it’s used up and then there’s 33% tax on the excess.