  We present the photometry of 16 91T/99aa-like Type Ia Supernovae (SNe Ia)
observed by the Las Cumbres Observatory. We also use an additional set of 21
91T/99aa-like SNe Ia and 87 normal SNe Ia from the literature for an analysis
of the standardizability of the luminosity of 91T/99aa-like SNe. We find that
91T/99aa-like SNe are 0.2 mag brighter than normal SNe Ia, even when fully
corrected by the light curve shapes and colors. The weighted root-mean-square
of 91T/99aa-like SNe (with $z_{CMB}>0.01$) Hubble residuals is $0.25\pm0.03$
mag, suggesting that 91T/99aa-like SNe are also excellent relative distance
indicators to $\pm$12%. We compare the Hubble residuals with the
pseudo-equivalent width (pEW) of Si II $\lambda\lambda$6355 around the date of
maximum brightness. We find that there is a broken linear correlation in
between those two measurements for our sample including both 91T/99aa-like and
normal SNe Ia. As the $pEW_{max}$(Si II $\lambda\lambda$6355) increasing, the
Hubble residual increases when $pEW_{max}$(Si II $\lambda\lambda$6355)$<55.6$
\r{A}. However, the Hubble residual stays constant beyond this. Given that
91T/99aa-like SNe possess shallower Si II lines than normal SNe Ia, the linear
correlation at $pEW_{max}$(Si II $\lambda\lambda$6355)$<55.6$ \r{A} can account
for the overall discrepancy of Hubble residuals derived from the two subgroups.
Such a systematic effect needs to be taken into account when using SNe Ia to
measure luminosity distances.
