I can see where one could still "see" the stigma of "widows marrying again" in society today. My Mom asked me once while we were out if it was ok for Her to dance with another man and she even said something along the lines that my Dad was the only Man for her. I got confused for a second and than I told her that Of course that would be ok because I completely understood her pain and her wanting to dance with someone. It was the sweetest thing watching my Parents dancing. They were so very much in LOVE and married for over 30 years. They were each others "Love of their Lifes" and this is my Moms decision to make if she wants to marry again or not.
I guess what I am getting at is talk with your Sister FIRST! Your sister may or may not want to get right back into a relationship. Everyone takes different amounts of time to "get over" situations that happen in life. Maybe, instead of pushing your sister into another relationship, maybe You could focus on the relationship You two have. What it your sister just wants to focus on herself and figure out what her Dreams are now, before she was with her husband and they had "together" goals and dreams, maybe now since he passed (my condolences) she wants to go to Europe and study art or learn to surf and travel the world circuit, this list is endless with possibilities. The point here that I am trying to convey is communication is always the key, talk to your sister. Most likely she may take longer than you think to figure things out. JUST BE THERE FOR HER and help anyway possible with HER decision, not what you think she should do.