the wives of maternal and paternal uncles are not considered Mahrams to their nephews; it is, thus, impermissible to sit in Khulwah with them or shake hands with them, let alone kissing them. In fact, they should wear Hijab (veil) in front of them. However, there is no harm in greeting them verbally without shaking hands, provided that there is no fear of Fitnah (temptation).
a sister-in-law is not considered a Mahram to her husband's brother; she should wear the Hijab in front of him; and it is impermissible for him to sit in Khulwah with her or visit her at home without a Mahram. The Prophet (peace be upon him) said:
"Beware of getting into the houses and meeting women in seclusion. A person asked, 'What about an in-law?' Whereupon he said: An in-law is like death."
(Related by Al-Bukhari and Muslim)
In-laws refer to husband's relatives, such as his brothers, nephews, or cousins. (Part No. 17; Page No. 434) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.