i only have ones i have already put on my reclist, so if you've already gone through those sorry for repeating! but here:

• I May Be Your Big Brother But I Want You To Be Sweet On Me / Onii-chan Datte Amaetai Wake de
• my brother's sweetest spot / Aniki no Ichiban Oishii Tokoro (twins, but bottom is "older")
• lotus flower in the mud / Deichuu no Hasu (darker manga)
• dear brother / haikei, niisan-sama (non-explicit)