I have 2 questions.
1. is there room for leniency with yichud and negiah, with an adopted sister who was adopted at the age of 5 months old, and grew up together with us as an entire family and with the normal love between siblings. She recently converted to Judaism as she is now 18. I am 7 years older?
2. How does someone with a non Jewish father sign as an ed on a kesuba or get. i have been told once to sign as ben my mothers name, and another time I was told to sign ben my mother bas my grandfather? Which way of these two, or any way is the correct way. Thank you.
1. In principle, although you grew up together, it is hard to permit Yichud with an adopted sister.
Many poskim discuss Yichud with an adopted daughter, and this involves a dispute among poskim. Even those poskim who are lenient, however, qualify their rulings: Rav Moshe Feinstein (Iggros Moshe Even Ha-Ezer 4:64:2) and Rav Eliezer Waldenburg (Tzitz Eliezer 6:40:21) are lenient in principle, yet Rav Moshe writes that his leniency does not apply to an adoptive father and daughter if the father’s wife has died.
Rav Chaim David Halevi (Asei Lecha Rav 3:39) is also lenient, and Rav Ovadia Yosef (see Yalkut Yosef, Kitzur Shulchan Aruch p.975) leans towards leniency, but a number of authorities are stringent: See Chazon Ish (cited in Dvar Halacha 7:20) and Rav Shmuel Wosner (Teshuvot Shevet Halevi 6:196) and see also Nishmas Avraham (5:132-133). A letter printed in Otzar Haposkim 9:130 presents the stringent opinion of the Lubavitcher Rebbe, an opinion that the Chebiner Rav (9:132) and others agreed to.
Concerning parents there is a special reason to lean towards leniency, because the stringency of Yichud can create very awkward circumstances. With regard to siblings this is not the case (to the same extent), and therefore one should be stringent for the Yichud prohibition.
However, because of the special nature of the sibling relationship, one can rely on lenient opinions for specific questions of Yichud. Thus, one can rely on those opinions who write that if the two are in separate rooms, and the sister’s room is locked, the prohibition does not apply (the leniency is noted by the Divrei Malkiel Vol. 4:102 and by Dvar Halachah 14:10 ft. 22 — though Iggros Moshe and Chelkas Yaakov do not concur).
If there are other specific questions please ask again.
In the same sense, one should be stringent concerning kissing and hugging — indeed, even concerning a blood sister the Rambam and later authorities write that it is proper to avoid these practices — but you do not have to be stringent to avoid any contact at all.
2. The mother’s name should be used. See Harei Besamim (Tinyana 172) who writes that for purposes of documents, where there is no concern for a person’s possible shame, the mother’s name should be used. He cites numerous precedents for this ruling.