A judge is being asked to rule on whether the will - which would indirectly benefit Deirdre Romanes's former husband and business partner - should stand.
Mrs Romanes's sister, Elizabeth Smyth, claims that businessman Iain Romanes was able to exert undue influence on his ex-wife when she "lacked capacity."
Mrs Smyth, of Kells, County Meath, Ireland says the will cuts her share by some 40 per cent, compared with a will drawn up two years earlier.
The later will, signed days before 60-year-old Mrs Romanes lost her long battle with cancer in May 2010, left the bulk of her fortune to a trust fund, an arrangement which would also benefit her former husband.
Judge Lord Glennie is being asked to set aside the later will because Mrs Romanes was not fully aware of what she was doing.
The hearing continues, with a probability that Lord Glennie will give a ruling in writing at a later date.