Partner with kids already?

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My husband and I have applied too have a dna test like advised we have an appointment tomorrow morning to have it done at my local health centre. Deffo mixed feelings about this🤷‍♀️

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So I met my fiancé a year ago and we both fell in love instantly. So jump forward to now and we’re engaged and neither of us could be happier. We fount out a week ago that he had a kid from a past relationship, this women had used him for money and sex and for a baby, he had no idea and she came knocking on my door at 10 pm demanding we take the kid who’s 5 and we look after him. My fiancé was dumbfounded he had no idea. He’s not even on the birth certificate and this women expects him to take him on because she made a mistake having him and wishes she could return to her old life. I am beyond pissed. I don’t know what to do or how to process it. I don’t want to turn the kid away because his mum ain’t the best caretaker and I want to live my life with my fiancé. Am I being selfish by not wanting the kid to live with us. Like we arnt exactly poor but I don’t want to spend my hard earned money on someone’s child. I want my own children eventually and not yet for a reason.