expression
if you insist
A set phrase used when reluctantly agreeing to a request after the other person has strongly insisted. Often carries a nuance of 'if you really must' or 'if you say so'.
どうしてもと言うのなら、手伝ってあげてもいいよ。
If you insist, I can help you.
どうしてもと言うのなら、もう一度だけチャンスをあげよう。
If you really must, I'll give you one more chance.
どうしても means 'no matter what' or 'by all means'; どうしてもと言うのなら is a conditional phrase built on it, meaning 'if you say you must'.
A fixed expression combining どうしても (by all means, no matter what) with と言うのなら (if you say that...). The exact historical derivation is uncertain, but it is a conventionalized phrase.