One of the main issues here is the family member living in her house. A new bank looking to lend against the property would not want any complications if they need to repossess the house, in the event of non-payment of the mortgage. A family member with a right to residency - especially an elderly family member - would be enough to put most banks off.
It's possible that one or two banks may consider lending against the house if the residing family member signed a waiver to any right to residence, but there would still be the issue of funding the increased mortgage, as well as the new mortgage you are planning to take out. It would not be reasonable to suggest that the property is being rented out currently and into the future, unless there is rent to show, so your incomes would need to support three mortgages with rental income on only one. Technically possible, but it looks very difficult.
Best Regards,
Dave Curry, Irish Mortgage Corporation
https://ie.linkedin.com/in/davecurryirl (LinkedIn profile and client testimonials)