When your father died your mother automatically became entitled to the whole of the house (unless its value exceeded £300,000) as she had 'prior rights'. (She was also entitled through her prior rights to all of the furnishings and furniture, as long as their value didn't exceed £24,000, together with the first £42,000 from the estate. She was then also entitled to one third of the 'moveable estate' under her 'legal rights').
If your father's estate was particularly large it's possible that your mother might have taken control of something to which she wasn't actually entitled; in which case that error would have to be corrected upon her death.
However, if we can assume that there are no such problems, then (upon the death of your mother) you and your siblings automatically get to share one half of her 'moveable estate' (consisting of things like money, shares, cars, furniture and jewellery) under your joint 'legal rights' but you'll also then get equal shares in her 'heritable estate' (meaning the house and any other land or buildings that she owned) together with the residue of the estate.
So, unless there are any complicating factors, you and your siblings will get equal shares in her house.
For more detail, start here
http://www.scotland.gov.uk/Publications/2006/04/12094440/20
and keep clicking 'Next'.