I had a June baby and then a Janurary baby. I re-used a lot of clothes, mostly sleepers at first. My second baby was a bigger baby though so she fit into bigger sizes sooner, like she was 3-5 months and fit into 6-9 months which was what my oldest was in the winter. I did still buy a lot og clothes at first because I wasnt sure on sizing. Now they are like 1 size apart and some things they are the same size. I still save my clothes from oldest and my youngest then usually can wear it at least part of whatever season the next year. I would save the clothes still or let your brother borrow but say you need them back .
Summer babies still wear sleepers. My DD1 wore them all the time in the summer when we where at home. We lived in my parents basement at the time so it was always cold plus having the A/C on made it too cold for her and she could regulate her temp to keep her from freezing.