Is this a Illegal Egg Move Combination?
Yawn, Mirror Coat, Sludge and Brine.
How can I also check whats a Illegal Move Combination and what isn't?
None of them are illegal
Source: Shellos Egg Move Page
You can tell if a Pokémon's Egg Moves are illegal or not on the above link. If you're looking for another Pokémon's Egg Moves, change the Shellos at the top of the page to the Pokémon whose Egg Moves you're looking for
Hope I Helped!!