Death - souls of relatives

One of the distinguishing characteristics of this most great Dispensation is that the kin of such as have recognized and embraced the truth of this Revelation and have, in the glory of His name, the Sovereign Lord, quaffed the choice, sealed wine from the chalice of the love of the one true God, will, upon their death, if they are outwardly non-believers, be graciously invested with divine forgiveness and partake of the ocean of His Mercy. This bounty, however, will be vouchsafed only to such souls as have inflicted no harm upon Him Who is the Sovereign Truth nor upon His loved ones. Thus hath it been ordained by Him Who is the Lord of the Throne on High and the Ruler of this world and of the world to come.

Bahá’u’lláh, The Compilation of Compilations vol. I, p. 386

As the spirit of man after putting off this material form has an everlasting life, certainly any existing being is capable of making progress; therefore it is permitted to ask for advancement, forgiveness, mercy, beneficence, and blessings for a man after his death, because existence is capable of progression. That is why in the prayers of Bahá’u’lláh forgiveness and remission of sins are asked for those who have died.

‘Abdu’l-Bahá, Bahá’í World Faith, p. 329

Concerning your question whether a soul can receive knowledge of the Truth in the world beyond. Such a knowledge is surely possible, and is but a sign of the loving Mercy of the Almighty. We can, through our prayers, help every soul to gradually attain this high station, even if it has failed to reach it in this world.

Shoghi Effendi, Lights of Guidance, p. 204-205

The Master has told us that gifts and good deeds done in memory of those who have passed on, are most helpful to the development of their souls in the realms beyond.

Shoghi Effendi, Lights of Guidance, p. 204

We cannot take the teaching that if a believer remains steadfast till the end of his life he will be the means of spiritual awakening of all his ancestors too literally, especially as it is only reported, and not in an authenticated form. However, we can be assured that the true and faithful believer will be in a much better position to intercede for his ancestors, and aid in their development than if he were not spiritually enlightened.

Shoghi Effendi, Lights of Guidance, p. 228

Your contribution to the Shrine of the Báb is very deeply appreciated... Your mother and father will both be helped spiritually to a very marked degree by this contribution in their memory, for the Shrine. Parents are glorified by the deeds of their children; and your service and dedication to the Faith will be the means of their spiritual development in the realms beyond.

Shoghi Effendi quoted in The Universal House of Justice, 1991 Aug 28, The Condition of non-Bahá’í Relatives after Death