The momentum distribution and atomic kinetic energy of the two isotopes of helium in a liquid mixture at temperature T = 2 K are computed by quantum Monte Carlo simulations. Quantum statistics is fully included for 4He, whereas 3He atoms are treated as distinguishable. Comparison of theoretical estimates with a collection of the most recent experimental measurements shows reasonable agreement for the energetics of 4He and pure 3He. On the other hand, a significant discrepancy (already observed in previous studies) is reported between computed and measured values of the 3He kinetic energy in the mixture, especially in the limit of low 3He concentration. We assess quantitatively the importance of Fermi statistics and find it to be negligible for a 3He concentration ≲20%. Our results for the momentum distribution lend support to what was already hypothesized by other authors, namely, that the discrepancy is likely due to underestimation of the 3He kinetic energy contribution associated with the tail of the experimentally measured momentum distribution.