Lots of ways to do this.
In base R:
myData <- myData[!is.na(myData$twin),]
In tidyverse:
require(tidyverse)
myData %>%
filter (!is.na(twin)) -> myData
Bear in mind - both are over writing your dataset
is.na() - means the value is NA
! - means IS NOT.
so !is.na() means IS NOT NA