Is it permissible for me to travel to Malaysia and leave my sister, who suffers from self-reliance issues, especially since my non-Muslim parents entrusted her to my care?

It is permissible to remain in a non-Muslim country for someone who embraced Islam there if he can openly practice his religion and is safe from trials (fitna). Emigration (Hijra) is recommended if he finds a better place to practice his rituals. His stay might even be better if he is engaged in inviting people to Allah. Emigration becomes obligatory for someone who cannot openly practice his religion or fears trials, provided he is able to emigrate. There is no blame upon you for going to Malaysia and living with your son there, and you are not obligated to stay with your sister. However, you should maintain contact with her, inquire about her, and visit her as a way of maintaining kinship ties.