  A novel practical few-body method is formulated to include isospin symmetry
for nuclear halo structures. The method is designed to describe beta decay,
where the basic concept of isospin symmetry facilitates a proper understanding.
Both isobaric analogue and anti-analogue states are treated. We derive general
and explicit formulas for three-body systems using hyperspherical coordinates.
The example of the beta decaying $^{11}$Li ($^{9}$Li+$n$+$n$) is chosen as a
challenging application for numerical calculations of practical interest. The
detailed results are compared to existing experimental data and good agreement
is found at high excitation energies, where the isobaric analogue and
anti-analogue states are situated in the daughter nucleus. An interpretation of
the decay pattern at lower excitation energies is suggested. Decays of the
$^{9}$Li-core and the two halo-neutrons are individually treated and combined
to the daughter system with almost unique isospin, which we predict to be
broken by about $0.4\%$ probability. Properties of decay products are predicted
as possible future tests of this model.
