OK, let's take this in stages.
1) You should probably sell House 2 now, even if there is a shortfall of €10,000
It's not a profitable investment - the rent is just about covering the interest.
From the lender's point of view, you will be seen as having two mortgages. I doubt you will be able to move home while having this second mortgage in your name.
It's costing you in cash flow.
There is a risk that it could damage your credit rating, if it has not already done so.
Getting rid of it simplifies matters a lot.
Given that your brother is unemployed now and can't contribute to the shortfall, maybe you should simply agree to pay the shortfall.
2) There might be an argument for keeping House 2
What is your brother's employment and financial outlook?
He will probably not be able to get loan approval to buy a house for some time.
If his finances improve, he may be able to return to paying this mortgage on his own.
3) Can you move on from House 1?
Who is your lender? The main banks allow you to move your tracker mortgage, with conditions - BoSI and Danske do not.
Even if you are allowed to move your tracker, you should stay where you are as long as possible as most of the repayments are going to capital repayments. When you do move you will have much more equity in the house.
Has your second mortgage damaged your credit rating? Check your ICB record to see what it says.
You will need a deposit of 20% of the price of a new house. Do you have that? If you want to buy a house for €400k, you will need €80k. You have about €50k equity less €10k to pay off the shortfall on House 2. So do you have another €40k cash?
Brendan