Principle of territoriality under the Revised Penal Code

Dear PAO,

My sister got married last year in the US. Unfortunately, after only a couple of months, she learned that my brother-in-law has a mistress whom he lives with in another State whenever my sister has weeks-long duty at work. My sister is thinking of going back home here in the Philippines because she has been under a lot of stress with what is happening in her marriage, but she also needs to stay in the US because of her work as a medical frontliner. Can she file a case for concubinage here assuming she proceeds with her plan of coming back? She and my brother-in-law are still both Filipino citizens. Please advise.