How will I cheer up my sister who already started mix feeding her 9mon old boy because she decided to only latch him at night time and use feeding bottle during daytime. Poor sister cries during latch as baby bites her and leave a super ouchy wound she can't stand it. :(

This is what you call nipple confusion. Babies younger than 3 weeks old and those who have just started their bottle feeding would have this as one of their issues. Good thing is the baby still latches on your sister unlike others, they try to refuse when they were already used to bottle feeding. Resolving this problem comes with a lot of patience. The best way to help your sister would be to find a breastfeeding counselor to guide her through this adjustment. Another is to use nipple shields for her comfort.