On 19b in berachos on the bottom I think the shaklah v tariah says that a nazir can't mtamei himself for his sister. But if he were to have the oppurtunity to do korban pesach he can't do it because he can be metamei meis mitzvah. My question is the gemara says he can't do bris milah because he may be tamei because of meis mitzvah? One can't circumcise his son as a tamei meis?
Tum'ah is not the problem. Rabeinu Chananel (Sanhedrin 35a) explains that by taking care of the Meis Mitzva there is no time left to do the Bris Milah on the eighth day- and still Meis Mitzva is more important (Rashi's 2nd explanation adds thatwithout the Bris the Pesach cannot be brought)
All the best,
Reuven Weiner