Question regarding the firstborn inheritance

Hello Rabbi Avraham,
I wanted to ask you a question about the birthright.
My late father passed away about ten months ago and left a respectable inheritance to me and my older brother. I converted to Judaism a little over a year ago, and my brother is still secular (for now). Six months ago we went to a lawyer to issue an inheritance order in a civil court (not a rabbinical one), and the inheritance order states that the property is divided 50/50. I found some kind of responsa online that says that if a first-born has voluntarily agreed to litigate in the courts, he is bound by their ruling. Below is the link to the responsa:

http://www.din.org.il/2011/02/20/%D7%90%D7%97-%D7%95%D7%90%D7%97%D7%95%D7%AA-%D7%A9%D7%94%D7%95%D7%A6%D7%99%D7%90%D7%95-%D7%A6%D7%95-%D7%99%D7%A8%D7%95%D7%A9%D7%94-%D7%91%D7%A0%D7%99%D7%92%D7%95%D7%93-%D7%9C%D7%A8%D7%A6%D7%95%D7%9F/

My question is whether I can rely on this and say that according to this, the eldest son waived his share of the birthright by agreeing to the issuance of an inheritance order in a civil court.

I should point out that my brother is unaware of the issue of the birthright inheritance, and does not demand it, but I am finding this out to know if I should bring him his share of the birthright, even if he does not recognize his right.

Furthermore, I wanted to add in this context that I have read in several places that it is possible to argue that Kim is of the opinion of the jurists who believe that one should not be forced to sign without compensation,
And another thing, those who believe that the debtor must sign the law of restitution of loss, then the law of restitution of loss for a secular person or for someone who is not in the habit of returning losses is questionable.

Best regards,