  Understanding Type Ia supernovae (SNe~Ia) and the empirical standardisation
relations that make them excellent distance indicators is vital to improving
cosmological constraints. SN~Ia ``siblings", i.e. two or more SNe~Ia in the
same host or parent galaxy offer a unique way to infer the standardisation
relations and their diversity across the population. We analyse a sample of 25
SN~Ia pairs, observed homogeneously by the Zwicky Transient Factory (ZTF) to
infer the SNe~Ia light curve width-luminosity and colour-luminosity parameters
$\alpha$ and $\beta$. Using the pairwise constraints from siblings, allowing
for a diversity in the standardisation relations, we find $\alpha = 0.218 \pm
0.055 $ and $\beta = 3.084 \pm 0.312$, respectively, with a dispersion in
$\alpha$ and $\beta$ of $\leq 0.195$ and $\leq 0.923$, respectively, at 95$\%$
C.L. While the median dispersion is large, the values within $\sim 1 \sigma$
are consistent with no dispersion. Hence, fitting for a single global
standardisation relation, we find $\alpha = 0.228 \pm 0.029 $ and $\beta =
3.160 \pm 0.191$. We find a very small intrinsic scatter of the siblings sample
$\sigma_{\rm int} \leq 0.10$ at 95\% C.L. compared to $\sigma_{\rm int} = 0.22
\pm 0.04$ when computing the scatter using the Hubble residuals without
comparing them as siblings. Splitting the sample based on host galaxy stellar
mass, we find that SNe~Ia in both subsamples have consistent $\alpha$ and
$\beta$. The $\beta$ value is consistent with the value for the cosmological
sample. However, we find a higher $\alpha$ by $\sim 2.5 - 3.5 \sigma$. The high
$\alpha$ is driven by low $x_1$ pairs, potentially suggesting that the slow and
fast declining SN~Ia have different slopes of the width-luminosity relation. We
can confirm or refute this with increased statistics from near future
time-domain surveys. (abridged)
