How to approach KBC
As you have no arrears at present and as their investment mortgage is safe, there is no incentive on KBC to talk to you.
You need to get your brother's agreement to take over the full mortgage. He might not like it, but that is the way it goes. He is effectively a creditor of yours like KBC.
You will have to pay a PIP and/or the costs of bankruptcy, so you notify KBC that you will be stopping paying your home mortgage to build up the funds for this. Don't stop paying the investment property as this just makes it worse for your brother. After you have built up a fund of €5,000, resume paying your mortgage on an interest only basis which should be around €1,100 a month.
At that stage, I think that KBC will see sense and will agree to your brother taking over the mortgage. They should also agree to some form of split mortgage in which case you will happily release back to them the €5,000 pot you have built up.
Alternative solution
If you are willing to give up your home, and you should be, then you seek KBC's permission to sell your home as well.
After that you apply for a 3 year Debt Settlement Arrangement for the shortfall.
If they veto it, go bankrupt.
Where will you live if you sell your home?
Is your brother's investment property suitable for you to rent? This could solve almost all the problems.