Umesh - the bank will most likely need to consent to the transfer so check both the register and the T&Cs of the mortgage itself.
If they consent then she can t
ransfer the whole of the legal ownership to herself and her children as appropriate. The online guidance explains how the joint ownership/shares can then be handled and protected.
See also j
oint property ownership and
PG 24 for guidance to help understand how such interests can be protected