Replacement effects that modify creatures of a certain type as they enter will apply after you apply Rin and Seri, Inseparabler’s first and second abilities, as appropriate. For example, if you control Rin and Seri, Inseparabler as well as a permanent with “Each Dog you control enters with an additional +1/+1 counter on it” and you create a 1/1 Cat token, that Cat token will be a Dog Cat token and will enter with a +1/+1 counter on it, a woof, and a purr.