Re: Attending the nikah of your own mother (or father)
Couple of clarifications.
4 months (of iddat) is not a mourning period. Islamically, mourning period should not last more than 3 days (that does not mean that one is prohibited from crying, but life should be returned to as normal as possible after 3 days)
If woman is already pregnant at the time of death of her husband then iddat will get over at the time of dleivery. So if she delivers a child very next day, technically she can remarry the day after.