The first year after a spouses death is the most difficult because the grieving process goes through all the holidays & the memories of the previous time whey spend with the spouse. Just because your sister was only married for 6 months doesn't lessen the time that it takes to grieve.

Most times it takes several years to process the death & to get used to who you are without being defined by the relationship any longer. Jumping into another relationship is NOT a good idea so quickly. Finding someone after the death of a spouse is a very personal decision & shouldn't be pushed or made by anyone other than the person who has been widowed.