Formal:
* I sometimes wish I had more siblings.
* I long for a larger family.
Informal:
* I wish I had more brothers/sisters.
* I wish I had a bigger family.
* It would be cool to have more siblings.
* I kinda wish I had more brothers/sisters.
Emotional:
* I really wish I had more siblings.
* I miss having more brothers/sisters.
* It makes me sad that I don't have more siblings.
You can also use phrases like:
* I wish I had someone to share my childhood with.
* I wish I had more people to call family.
The best way to say it depends on the context and your relationship with the person you are talking to.