How can I help my niece talk to her Mom about this?

My sister lost her husband a year and a half ago from stomach cancer, he was 26. they were together for 9 years and had 2 children. not even like a few months after she started dating this guy. I mean dont get me wrong hes really nice but I think it was too soon. she went threw this phaze where she was really depressed and started going out to bars and stuff and my sister never even been to a bar before. maybe like once? she was always at home with her family. well I was talking to my niece who was 9 years old and her mom went out with her boyfriend and the kids stayed at my house. I could see the look in her eyes that she was hurt. I felt stupid saying something to a nine year old, but she doesnt have anyone to talk to and she doesnt like to talk to her mom about it. after a while of telling her that she can always talk to me no matter what, she told me that she didnt like the boyfriend and she wished her mom never started to date. I started to cry, what was I suppose to say to a little nine year old about her mother dating after her father just died? it hurts me everytime I see this little girl because I know she wants to say something to her mom but she cant. I mean I promised I wouldnt tell my sister. How can I help my neice talk to her mom about this?