Right. No guarantees so make copies of any file you are going to change first!
There are a few options depending on whether you want merely to give your daughters a mother or want to make her your lord's wife as well, also whether you wish to modify a save game or a starting scenario.
Whatever you do, you want to find your daughters a mother. You can create a new character if you want but it is probably better to find a suitable character of the correct age by looking through the game portraits. I imagine what you want to find is an ancestor of an in game character (or an ancestor's sister) who died at around the right time (after the birth of your second daughter and before you married the wife you assassinated) and whose earlier husband (there will almost certainly be one) died before your eldest daughter was born; more than nine months before would raise fewer eyebrows!
If you find an in-game character then make a note of thier first name and their birth and death dates. Whilst you are at it, make a note of the names and birth dates of your lord and his two eldest daughters.
Now go to the game files.
DVIP may well change file locations and these change between operating systems, but in my installation on Windows 7 64-bit, the starting scenarios are in
C:\Program Files (x86)\Paradox Entertainment\Crusader Kings\scenarios
and the save games are in
C:\Users\[username]\AppData\Local\VirtualStore\Program Files (x86)\Paradox Entertainment\Crusader Kings\scenarios\save games
I am sure you can work out which files relate to the save you wish to edit.
It is easy to give your daughters a mother.
In a starting scenario you want to modify the file 1066_scenario_characters.inc (or similar). In a save game you want to modify the .eug file. Remember to save a copy first, then open in Notepad.
Search (ctrl-f) through the file looking for your new mother's name. There will probably be several matches but with any luck only one with the correct birth and death dates. The format is a little different in starting scenarios and save games; the latter shows dates as "yyyyddmm".
Make a note of her type and id values, eg id = { type = 10 id = 109625 }.
If you are going to make her your wife, also make a note of her actual death date.
Now hunt for your two daughters in the same way. Once you have found each, just copy the father line to a new line below, change father to mother and the type and id to the numbers you noted earlier.
Easy, eh?
To marry your count to your daughters' new mother you will need your count's type and id. You can take these directly from your daughters' father entry.
The next step depends on whether you are modifying a starting scenario or a save game.
For a starting scenario, you can save and close the file you have been editing and open instead 1066_scenario_relations.inc (remember to create a copy first). For a save game you will continue to edit the same file.
Find the list of marriages by searching for "marrige" (yes, it really is spelt that way!). There will be lots of them with variations in layout.
Pick a complete entry such as
Code:
marrige = {
primary = { type = 10 id = 472 }
secondary = { type = 10 id = 479 }
startdate = { year = 1059 month = january day = 1 }
enddate = { year = 1065 month = january day = 1 }
}
and copy and paste it so it appears twice. Check it looks right because that is the best clue you have the formatting correct.
Now change one of the entires so primary is your count's type and id, secondary is your new wife's type and id, start date is the day they got married and end date is the date she died.
Save and close.
I admit I have not tried this out myself, but you did save those copies of the original file(s) didn't you?