Would you marry a man/ woman with children in the past relationships?

I have a sister and she has a boyfriend (her first bf). They are like 2 years together now and physically away from each other right now. (She's in the UK and guy is from somewhere).She has a "motive" of getting married to his boyfriend although he already has a child from past relationship. As a woman, would you still marry a guy who already made some girl pregnant?

Add ons: My sis is actually doing good in life now (like financially), while the guy is not. Im just worried he might just be using her or something? I know I sound paranoid, but it's her first bf that's why she's crazy about it, but still, how do I make sure? She swears she will support our parents, but when she gets married, I dont think that would be the case. She's 24 and the guy is 29 I think.