I heard that 2 married sisters should not live in the same building. I live in Jerusalem and have the option of living in an apartment in the same building of another sister. Is it better not to? What is the basis for this?

ב"ה Shalom The source of what you heard is from the "Tzava'at Rav Yehuda Hechassid" who lived in Germany at the end of the 12th century. He wrote a will and testament in which sets down certain prohibitions based on more mystical traditions, which are not mentioned in the Gemara, which many Jewish communities have been keeping over the centuries. Some of the more popular ones are that one should not marry a woman who has the same name as his mother, nor should his name be the same as the father of the bride. Just as many who adhere to the special traditions there are those who have said that to begin with these traditions were not meant for everyone but only for the family of Rav Yehuda Hechassid. (נודע ביהודה תנינא אבן העזר סי' ע"ט). What it actually says in the Tzava'ah (#33 and later in the sefer #247)) is that two married brothers should not live in the same city. Rav Margaliot zt"l in his notes on the sefer called "Mekor Chessed" writes that this restriction is only for two brothers who are also "Nesi'im" princes or leaders. Also stated there, that this restriction only applies in a place where the brothers are new to the place because of "Ayin Hara" , however if they grew up over there and people are used to seeing them one does not have to be strict about it. So on the one hand the are reasons to be lenient, but on the other hand if the issue is "Ayin Hara" perhaps it would be a better idea not to be in the same actual building. In addition, since people may have also developed their own traditions around this and widened its original scope, whether justified or not, others may not feel comfortable with this situation so it would be best to ask the local Rabbi how things are done in the neighborhood. After completing this answer, I found a Teshuva written specifically about this question in the Sefer Yashiv Yitzchak , by Rav Yitzchak Shechter who is Tzanz chassid and an important posek, who has already published 44 volumes of Teshuvot=answers in Halacha. (שו"ת ישיב יצחק חלק מ' יור"ד סי' י"ז) He writes that on the one hand even though Rav Yehuda HaChassid did not write about two sisters, there are still those who are of the opinion that two married sisters are included in this restriction. On the other hand, he writes that the previous Tzanzer Rebbe o.b.m , wrote that in Eretz Yisrael one does not have to be concerned about two sisters living in the same building. He also adds that the Yismach Moshe and other Tzaddikim have said that if three Rabbanim are asked one may be lenient. In any case regardless of this addition, in order to make sure to avoid any dispute among neighbors and other tenants, ask the local Rabbi. All the best