Breastfeeding and expressing

Hi all. My little girl is 4 weeks old today. I am breastfeeding and having no troubles what so ever and really enjoying my experience with it. However I would really like my step son to be able to give his little sister a feed. The only issue with that is I don’t want her to get used to bottle feeding and then refuse to feed from me. What’s the best option? My midwife said to continue breastfeeding till she is 6 weeks and then express for him to be able to give bubs a feed. What’s the best option? Should I wait?