We strongly advise that divorced/widowed/older sisters need to find a Wali who will support them through out the whole marriage process from the start until the end (nikah).
The wali ideally must be the imam, the Islamic judge or a Muslim with some authority in her area.
The Prophet's (peace and blessings be upon him) hadith: "No marriage contract can be concluded without the presence of a wali. A Sultan (authority figure) can act as a wali for those without one." (Reported by Ibn Majah and Imam Ahmad)
Regardless of whether a sister is older, a divorcee with or without children she should not be communicating with a brother one to one but rather keep her Wali involved and updated of the progress. This is no doubt for her protection and according to the Sunnah.