Mimi wasn't a small baby, she was 8lb 4oz at birth. The only pockets I had when she was tiny were bumgenius. I didn't try to put them on her until she was about 4 weeks old. They fitted her fine then, if I'd tried them earlier they'd have probably fitted her straight away. Everyone says that they aren't good for bf poo but we've never had a problem. They've always contained it well, ok not quite so good as 2 parters but that's still the case now. The only thing I did get was some wicking around the stretchy tabs. She liked to lie on her side and I'd sometimes find her wet.
Anyway, they convinced me to get lots more pockets. I'm sure some small, sized ones would be fine from an early age. Well done for converting your sister, no such luck with mine