I recently started wearing Niqab, Al hamdulillah. Insha Allah, I will be spending the summer with my sister and her husband. Do I have to keep my Niqab on in front of him at all times or is it sufficient to wear just my headscarf and regular Hijab? And also is it permissible for me to wear pants in his presence?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

You should cover all your body including your face from your sister's husband as he is a non-Mahram man to you.
In fact, it is forbidden for him to marry you as long as he is married to your sister. But it is lawful for the woman's husband to marry her sister if he divorces that woman or if she dies. Similarly, it is unlawful for you to wear pants in his presence or to stay alone with him. The Prophet said: “No one of you should stay alone with a (non-Mahram) woman as Satan will be the third one present.” [Ahmad]

Al-Munaawi said while explaining this Hadeeth: “Satan will do everything in its ability to arouse their desire and drive them to commit sins.”

It is also unlawful for you to talk with your sister's husband except under the conditions set by the Sharee’ah in this respect. Allah Says (what means): {If you fear Allah, then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech.} [Quran 33:32]

Ibn Katheer interpreted this verse by saying “What is meant here is that she should not address non-Mahram men in the same way she addresses her husband; i.e., talking without being lenient or too soft. She should not use soft words while talking with non-Mahram men.” [End of quote]

Allah knows best.