Would I be a good mother? Different opinions..

So my sister in law has a beautiful daughter and I love her very much. She is creative and silly and sassy and fashionable. The normal 4 year old. After my husband and I married and he left for deployment I used that year to get to know his family better. And we enjoyed each other for the most part. But my niece can really be a handful. She's an only child with no siblings in sight so she doesn't have many people to play with in a house full of adults. Whenever the family got together she would demand someone to play with her and throw fits when she wasn't getting enough attention. I would cave and play dolls and do puzzles with her but mostly I wanted to spend time with the family not hide away in her room playing Barbie's.

One day I was expressing my headache to my sister in law and I mentioned how I didnt want to play that day because I was so tired and aching. Honestly my headache was coming from her daughter whining and screaming. She commented that if I can't handle a kid's attitude then I'm not ready to be a mom.. And that sort of stuck to me as an insult but I got to thinking about it and wonder if I'm even mature enough to be a mom when we do get pregnant.

I love my niece, it's just that she was raised getting whatever she wanted because everyone felt sorry for her being the only child. She was SHOWERED on Christmas and her birthday. She would get gifts for no reason. And now she expects people to either give her something or okay with her for hours until she's tired or else she thinks you don't love her.

What do you think, is my sister in law correct?