Rav Sternbuch( 4:279): Should one avoid marrying a widow?
This that you ask regarding marrying a widow, we find in the holy Zohar in parshas Mishpatim about taking another wife -
Zohar (2:102a): What, then, becomes of the spirit of an ordinary man whose widow has married.again? Come ye and see the wonderful and mighty works of the Holy King! Who can utter them? When.the second husband's spirit enters into the body of the woman the spirit of the first husband contends with.it, and they cannot dwell in peace together, so that the woman is never altogether happy with the second.husband, because the spirit of the first one is always pricking her, his memory is always with her, causing.her to weep and sigh over him. In fact, his spirit writhes within her like a serpent. And so it goes on for a.long time. If the second spirit prevails over the first one, then the latter goes out. But if, as sometimes.happens, the first conquers the second, it means the death of the second husband. Therefore we are taught.that after a woman has been twice widowed no one should marry her again, for the angel of death has taken.possession of her, though most people do not know this. Friends, I am aware that on this point you may.well object that in that case the second husband's death was not in accordance with Divine judgement. It is.not so, however. It is all decided by fair trial, whether the one spirit should prevail over the other or be at.peace with it; but he who marries a widow is like unto one who ventures to.brave the ocean during a storm without a rudder and without sails, and knows not whether he will cross.safely or sink into the depths.
In Jerusalem many have the practice of marrying a widow only after they have done the Tikun of the Rashash. However in the majority of communities there is absolutely no concern about this and they marry directly without the Tikkun. Your question is whether one should in fact be concerned about this?
It would seem that in our days that there is a basis to be lenient in spite of the Zohar. The basis of the Zohar's concern is the joining of the souls of the husband and wife in the first marriage. That means that each one has kedusha created in his soul through their joining together in the first marriage. Consequently the Zohar writes that the soul of the woman now is bound together with the first husband. Consequently his spirit strongly impacts her even after he has died and therefore no one else should marry her. However it is known from the commentaries that today marriage is always considered zivug sheini (a second marriage). That is because we all are reincarnations from a previous existence since our souls have not achieved the proper perfection. Therefore the marriage is according to one's deeds and therefore it is alright to marry a widow and I have never heard anyone saying otherwise. In addition I have never heard or seen outside of Israel that anyone insisted on during the Tikun of the Rashah. Therefore it would seem that even according to the Zohar it is not necessary in our day since the nature of the marriage is different and therefore the reason for concern of the Zohar doesn't apply anymore. Consequently the poskim don't even mention this issue.
In fact it would seem that the main concern of the Zohar is when the first husband is a great talmid chachom and now the widow is marry a person who is truly ignorant of Torah. In that case the spirit of the first husband would disturb her and would not find peace. However if the second husband is a talmid chachom or at least a G-d fearing man who observes all the mitzvos properly - then this would not arouse the objection of the deceased husband. Because there is no question that we have no desire to keep her unmarried for the rest of her life. In fact I heard that the Gra married a widow and had no concerns. That is because the first husband obviously had no objection for his widow to marry such a great Torah scholar and in such a case the husband's spirit would not disturb his widow. Even though we in truth find kabbalists in mishnas chassidim who warned not to marry a widow (see Chavis Ya'ir #197) -nevertheless since the established practice is not to be concerned with this we say "G-d guards the fools" and one should not be concerned at all about this.
However since one can in fact readily take care of even this ignored concerned then I would advise that the second couple should request of the kabbalists in Jerusalem to do the Tikun of the Rashash - and this is the right way to act. If this is done there is absolutely no concern at all but they should get married and have a wonderful life together with G-d's help. The main thing is to direct ones deeds according to the Shulchan Aruch and poskim.
Furthermore the Chida writes that the Zohar's concern only apply in the first 12 month of the first husband's death and this is also mentioned in the Maharasham (2:141) who says he receives this from the Shinover. In the new Maaseh Rav of the Gra he writes, In the Zohar parshas Mishpatim we find that it is very strongly against marrying a widow. There is a leniency that after 12 months of the death of the first husband there is not so much of a danger since his spirit has left from there. And this is what I have observed amongst distinguished rabbis of our day. ... We see from there that as long as the second marriage be done for the sake of Heaven that a frum person has nothing to fear and there is absolutely no concern for this issue. Consequently according to all this if he has an alternative to marrying a widow even after 12 months perhaps it is best if he take it since he is protected only if he does it for the sake of Heaven. See there that some say that the concern for the spirit of the first husband returning is only for special individuals and very holy people but not for others. Some are insistent that on the day of the wedding to give charity for the elevation of the soul of the first husband and that is sufficient.
You should also be aware that in the case of marrying a widow I am very stringent to insist that the yichud should be in a room in which there is a bed so that it possible to have sexual intercourse. That is because a widow requires either intercourse or at least the chupah should be possible to actually have intercourse. I am also very insistent that the time of yichud should be extra long (at least 10 minutes) before others entere the yichud room. In this manner the chupah and marriage are absolutely not problematic. In contrast with a virgin it just depends on being secluded for a short time as is explained in Sotah (4b). ... and it is necessary to remain in the yichud room for the amount of time that sexual intercourse takes according to many poskim.