Over an algebraically closed field (of any characteristic), it is fairly obvious using the omitted definition of "standard parabolic" that the assertion here for a pair of included standard parabolics is true: one just wants to trace the pairs of positive and negative roots involved.    However, the question is too loosely formulated to be clear.   For example, the minimal $k$-parabolic $P_0$ is introduced but then ignored for $k=\mathbb{Q}$.  Here $k$ may be arbitrary, but it needs to be clarified what "standard" means in this setting.   The 1965 foundational paper by Borel and Tits is now somewhat old-fashioned in language, but the BN-pair setting for parabolics is probably helpful here.