How can I convince my sister to baby-led wean her 6mon old than feed him with instant cereals as his 'first food'?

You can show her some BLW youtube clips and convince her it can be done. Also, you can borrow BLW books from library for her to read up the reasons behind BLW and its benefits. Initally my husband also not convince to let me try BLW but I did the same as suggested above and finally he supported me through the journey.