Question

Are all Pokemon from the same evolutionary line always in the same egg group?

Answer 1

Im not sure black empolian understood what you said by what you asked im guessing that you meen lets say treecko grovile and sceptile are in the same evelutionery line they are in the same egg group.
im not totally sure i understand your question but do you meen like lets say torchic combuskin and blazkin (pretend that combuskins in monster group and torchic and blazkin are in earth group) there for they are an evolutionary line that are not in the same egg group because if thats the case i can easyly answer the question with
Yes some pokemon are in differant egg groups and in the same evolutionary line for example some pokemon in the undiscovered egg group.
take riolu in the undiscovered group it evolves into lucario in the feild group.

Answer 2

Usually yes, but there are a few exceptions:
Female Nidoran is in the Field and Monster egg groups, but Nidorina and Nidoqueen are both in Undiscovered.
All baby Pokémon are in Undiscovered, but none of their evolutions are.
* Nincada and Ninjask are in Bug, but Shedinja is in Mineral. Shedinja technically doesn’t “evolve” from Nincada (it appears in your team after Nincada evolves into Ninjask), which might explain this.

Outside of these examples, yes, Pokémon from the same evolutionary line always share an egg group.