I wander if we can use "which" or "whom" as object relative pronoun after indefinite pronouns. For example: don't tell her anything which you want to keep secret. I know for sure that we can use "that" or live it out in this sentence, but wat about "which"? Thank You.

As a general rule, with non-personal heads in restrictive relatives, it's a free choice between the wh type and the non-wh type. But with compound determinatives like "anything", "something", "everything", etc. there is a preference for the non-wh type. "Which" in your example is not ungrammatical, but I’d stick with a non-wh relative:

Don't tell her anything (that) you want to keep secret.


Same applies with personal heads: wh-relative okay, but non-wh preferred:

You can invite anyone (that) you like.


PaulM