User richard describes very well what's wrong with your
sed command, and also gives the correct editing script to perform the action you're attempting.
I'm giving an alternative approach which will work in a number of different cases:
r will insert the contents of a file at the current position in the data stream.
sed command line could be written
$ sed '115r /dev/stdin' index.html <data.in
which would have the same effect as
sed '115r data.in' index.html... if the data that you wanted to insert after line 115 was stored in the file
/dev/stdin is a special file that contains whatever is sent through the standard input.
You could also do (with shells supporting "here-strings"):
$ sed '115r /dev/stdin' index.html <<<"my string of stuff"
$ sed '115r /dev/stdin' index.html <<<"$myvariableofwonder"
This also works as expected with piping from other commands, obviously:
$ sed -n '1,10p' myfile | sed '115r /dev/stdin' index.html
This will transplant lines 1 to 10 of
index.html at line 115.