I'm with PTSB and I've lived in the house since it was built 9 years ago.
Sorry, I had misread your post. As the house is your Principal Private Residence, ptsb will allow you to port your tracker.
Although unlikely that the sister will go for that while its in NE.
Sorry, I don't understand what "that" is. You and your GF should be happy to take over the house and the mortgage from your sister without any financial adjustment. The cheap tracker compensates for the Negative Equity.
So my revised suggestions are as follows:
1) Seek your sister's agreement to take over the house and the mortgage without any financial settlement.
Meet with ptsb and seek their agreement for the following:
1) You and your GF get loan approval for €300k (or whatever figure you need.)
2) They allow you and your sister to sell the house and you carry the tracker over the new property in the name of you and your GF.
This would be easier and cheaper than your GF buying out your sister now.
Will you get approval for a Negative Equity mortgage from ptsb?
Have a read of this Key Post.
Analysis of new ptsb tracker mover
You will need 10% of the purchase price in cash i.e. €30k
If you buy a house for €300k, the resultant position will be:
House value: €300k
Less sale of existing house (€250k)
Plus existing mortgage €290k
Less 10% deposit: (€30k)
Resulting mortgage €310k
Loan to Value: 103% which is well within ptsb's guidelines.
Brendan