Check the Philippines column for F1 and F2B here -- https://travel.state.gov/content/travel/en/legal/visa-law0/visa-bulletin/2023/visa-bulletin-for-december-2022.html
Has your MIL filed the I-130 petitions for her kids yet? If not, the visa process for her kids will take at least 10 years, whether she becomes a US citizen or remains an LPR. If I were in her shoes, I would naturalize ASAP, so I can stay in the Philippines to be with my family for as long as I want, without worrying about losing my US LPR status.