I am sorry for your loss,
As i do not have a specific title register to consider the following is in general terms.
The legal ownership is always dealt with as a whole. It is the beneficial ownership that can be split in the way you described.
If the property is to be transferred then it would require a Transfer (form TR1) by your Mother to herself plus whomever needs adding. Our
online guidance explains what's required A joint proprietor restriction indicating that a trust is in existence is another option.
The reason I say 'If the property is to be transferred' is that it is always recommended that you all seek legal/financial advice before deciding on what to do. We deal with the outcome of such advice and once you have decided what's best.
Darren