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

Perhaps she prefers her child to eat in puree form first? Then you can suggest her to make fruit and veggies puree at early stage of weaning. Later she can add oat and rice cereals and porridge in the menu. Tell her that it is not compulsory to begins with rice cereal. It is also a chance for her child to discover new varieties of food, some has different colors and some has different tastes and textures.

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.

I highly recommend this book as it covered the complete knowledge in BLW and comes with recipes in stages.