"their names added to the deed of their father's house" - means - they receive the property as a gift.
The basis for the gifted property is the same as the donor (in your situation - the father) had at the time of gifting - that is mainly the donor's purchase price adjusted by any improvement and some other expenses.
Each sibling will report his/her share of the proceeds and basis and should calculate the capital gain on the schedule D.
Because they owned the property more than a year - that will be long term capital gain.
The father generally should report his share, but because he used the property as a primary residence - his gain is not taxable and generally he doesn't need to report it.
Let me know if you need any help.
If the house was purchased in 1973 - yes that purchase price would be the basis.
There should be improvements over these years - I suggest you to investigate.
At least I might guess that the roof was replaced at least twice...
Check if windows and/or doors were replaced?
Trees were planting?
The gift is not a taxable income and is not reported on the tax return.
there is a separate gift tax return that should be filed by the donor - in your situation by the father in 2005.
A recipient of the gift - does not need to claim it as income. Please see for reference IRS publication 525 - http://www.irs.gov/pub/irs-pdf/p525.pdf
The donor - if he/she is an US person - would be required to file gift tax return (form 709 - http://www.irs.gov/pub/irs-pdf/f709.pdf ) if the gift is more than $11,000 per person per year (for 2005).
There will not be any gift taxes unless lifetime limit of $1,000,000 is reached.
I will repeat my statement from above
They did not need to report the gift as income which they received in 2005.