In my personal experience, even if you're remortgaging your home (not sure from your post if you're financing on your home or your in-laws) , the bank deal with it as a new seperate mortgage, therefore it shouldn't effect your existing mortgage. However, see * below
Just don't expect to get the new mortgage on the same great terms as your existing tracker. The banks are no longer offering such good tracker loans i.e some banks are offering tracker loans but the tie-in rate to the ECB rate is higher (e.g you might be on ECB+1% on your existing mortgage but a new tracker mortgage for the extra €30k could be ECB+2%)
* You may need to check if; 1) by adding a new mortgage and 2) the reduction in value of your home may effect the loan to value (LTV) ratio to such an extent that it moves you into a different rate bracket (overall).
Example, if your home was valued at €400k and your mortgage was €200k your LTV would have been 50%. Now your home could be worth €350k and your mortgages are €230k your LTV is 66%.
Other cost considerations with the mortgage; your mortgage protection insurance will have to be increased to cover the additional €30k and you may incur solicitor fees
Best of Luck,
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