There is no perfection in planning for the future.
1.No guarantee the siblings will like each other>>some siblings quarrel and fight every day
2. No guarantee they will help each other or take care of parents. My hubby's brothers only borrow from him and never support their father financially. Only my hubby , the youngest son, does that.
3. No guarantee the second baby will be healthy. My friend's younger son is autistic and my uncle's younger daughter is crazy...more burden on the older child as an adult having to take care of 2 parents and a younger sibling
Life may not turn out the way you want. So just live today to the full and come what may.