Unfortunately, a step-child will not usually be able to act as Administrator.

There are strict rules about who can apply to be the Administrator. The following people are eligible to apply and are usually eligible only in this order of priority:

a) he married partner, civil partner or unmarried partner of the deceased;
b) A child over 18 of the deceased, or the children’s guardian if the children are minors;
c) A grandchild over 18 of the deceased;
d) A parent;
e) A sibling;
f)  A nephew or niece; and
g) Another blood relative or other person appointed by a court