I would echo the comments about keeping your guard up and not being cajoled, or manipulated into loaning or gifting money, since people only go to friends when the issue is beyond their control, and a traditional lender will not loan, or at a reasonable rate.
Credit card companies are known to check land registry, and upon discovering property ownership, can place a charge on it, without the cardholders' knowledge or consent. This can be so, even if the debt is being serviced. This allows them to pressure a sale, should these debts not be serviced.
From your comments, I sense a palpable denial on the brother's part-rather than desperation to address the issues at hand. Many years ago, I found myself (circumstantially, rather than by design) in considerable debt (£20k unsecured). In these circumstances, people get desperate and tend to drip-feed information, so as to maximise the probability of friends, family etc reciprocating, plugging holes, while not tackling the real debt and wider issues.
I was ultimately rescued by a relative on a once-only "full disclosure" deal i.e. detail everyone owing. The truth, the whole truth, and absolute truth, or so help you God basis essentially. Much of this arose from a loveless and controlling marriage and some bad decisions on my part, the sort that went firmly against my debt-adverse nature.
Something I had to confront at the close of my father's funeral. Having returned to the car, the first words from my then wife's mouth were " Did he leave you any money?" I went hold, cold, numb and largely silent, realising, beyond any doubt that I needed to leave.
There are several points here. Never lend or borrow money unless you are capable of repaying in a reasonable amount of time, or can afford to lose/gift I borrowed hundreds of pounds from friends and was ultimately able to repay, but far later than I'd intended. A recipe for embarrassment and strained relationships. These never fully heal.
People have to confront and actively want to change their circumstances. The brother and sister inn law seem unwilling to do so. They must be left to work a solution together themselves. Otherwise, this will also likely result in a divorce. They may be enabling each other and living in la-la land. Alternatively, your brother may have been pressured down certain financial avenues and "going along with it to keep a sense of peace". However, this doesn't seem so, from the spending habits described. Bottom line. Under no circumstances, no matter how much they plead, do not loan money, or be dragged further down this particular fiscal rabbit hole, for your sake, but also for his.