We present optical and near-infrared photometric and spectroscopic observations of the fast-declining Type Ia supernova (SN) 2015bo. SN2015bo is underluminous (M_B_=-17.50+/-0.15 mag) and has a fast-evolving light curve ({Delta}m15(B)=1.91+/-0.01mag and s_BV_=0.48+/-0.01). It has a unique morphology in the observed V-r color curve, where it is bluer than all other supernovae (SNe) in the comparison sample. A ^56^Ni mass of 0.17+/-0.03M_{sun}_ was derived from the peak bolometric luminosity, which is consistent with its location on the luminosity-width relation. Spectroscopically, SN2015bo is a cool SN in the Branch classification scheme. The velocity evolution measured from spectral features is consistent with 1991bg-like SNe. SN 2015bo has a SN twin (similar spectra) and sibling (same host galaxy), SN 1997cn. Distance moduli of {mu}=34.33+/-0.01(stat)+/-0.11(sys)mag and {mu}=34.34+/-0.04(stat)+/-0.12(sys)mag are derived for SN 2015bo and SN 1997cn, respectively. These distances are consistent at the 0.06{sigma} level with each other, and they are also consistent with distances derived using surface-brightness fluctuations and redshift-corrected cosmology. This suggests that fast-declining SNe could be accurate distance indicators, which should not be excluded from future cosmological analyses.