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I feel like my dad is only nice to me to get sex out of his wife
I really don't understand this part of your post. How is being nice to you, getting sex for your dad? I could see him getting sex by being nice to his new wife, but to you? That doesn't make sense.
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my mum rarely says I love you.
Rarely isn't never. My mom was of the old school belief that saying "I love you"all the time, meant nothing. Words are easy to say, showing someone you love them by what you do, how you act, means a lot more than words.
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I have cut before and my mother knows this and my dad is to blind to see anything wrong with me
This is a cry for help and if you're doing this it means mental issues that you cannot control on your own. You need mental help, may even hospitalization in a mental health facility.
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I'm scared to go to foster care in case my mum or dad get investigated.
You wouldn't be placed in foster care unless your parents were investigated and your home was found to be a danger to you and your siblings. In that case, if they did find that you're in an abusive home, or being sexually molested, mistreated, malnourished, etc. etc. you and your siblings would be removed from the home and placed in foster care.
You don't mention that your parents are abusive, or that you're being sexually abused, or that anything else is going wrong in the home. You do mention that your parents are fighting, and that you're not happy. That's not reason enough to remove you from the home.
I don't know enough to say with certainty what you should do. If I base my answer only on what you posted, and I'm sure that's not the whole story, I'd have to say that it sounds like you're the one with the issues, that you're depressed, unhappy, and need help dealing with that.