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We present optical and near-infrared photometric and spectroscopic observations of the fast-declining Type Ia supernova (SN) 2015bo. SN 2015bo is underluminous (M_B= −17.50 ± 0.15 mag) and has a fast-evolving light curve (Δm15(B) = 1.91 ± 0.01 mag ands_BV= 0.48 ± 0.01). It has a unique morphology in the observedV−rcolor curve, where it is bluer than all other supernovae (SNe) in the comparison sample. A⁵⁶Ni mass of 0.17 ± 0.03M_⊙was derived from the peak bolometric luminosity, which is consistent with its location on the luminosity–width relation. Spectroscopically, SN 2015bo 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)andsibling (same host galaxy), SN 1997cn. Distance moduli ofμ= 34.33 ± 0.01 (stat) ±0.11 (sys) mag andμ= 34.34 ± 0.04 (stat) ± 0.12 (sys) mag are derived for SN 2015bo and SN 1997cn, respectively. These distances are consistent at the 0.06σlevel 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.