Laura, your dad was obviously a weak man. Why else would he be able to sneak a way when his wife and child were at the shops, then to be so manipulative when he tried to come home again.
Your sisters mum - what makes you think she would want your sympathy or even need it? The woman seems to have managed to move on from your father and made a life for herself even to this day, and it may make you feel better to insinuate she's some kind of venus fly trap when it comes to men, men who have money, but she's had children with 3 men and is currently in a relationship with someone else. Do your really believe all of those 4 men were taken in by her, and all the more-so when each one would have been only well aware of her romantic history given the children who were in front of their nose? I think its more likely they new enough about her history to think there was nothing to be suspicious of when it came to being with her, even though things didn't work out for them as a couple long term either. It happens.
The men the woman has had in her life have been wealthy - yep if you mix in circles, and are accepted in circles where people have money its kind of a given that everyone you have contact with will be wealthy.
She is with her partner and going down a similar path - really? She must be some age now. Is pregnancy still possible for her? Or is it likely someone else has seen something in this woman and is willing to give it a go with her?
Were her other children taken away from her? No. I dont think they were. I think you've made this up. I feel very certain if they had been taken away from her you would have mentioned it long before you did and not just in a post tacked on to the end of another. I think you were angry at what I said because it gave you food for thought, and hurt you, and this was your retaliation.
Your anger at not having a sibling from your mum and dad - I'm sorry but there's is that need for 'one up' on her again. You want to be able to say, ah well, he came back to us and had another baby with my mum so here's more proof that we were the 'real' ones in his life. You have to understand that you'd still be angry if this had happened and you'd just have found something else to be angry about. Its the whole thing you're angry about, not just bits of it.
Paternity - you're dad accepted your sister as being his child and nothing you insinuate based on her having different colouring to you and him will change that. For five years they were all in touch, your sister more than likely called him daddy, and if your father doubted she was his then all the more reason for him to have just paid child support and never look back. Instead though he saw her and her mum for about 5 years. But that aside, there are 5 fingers on a hand and none of them are the same. Your sister may not look like you but she may have the same colouring as others in your family, she may even look like them, but more to the point, she may just take after her mums side of her family. Genetics are a funny old thing and can throw up many a surprise - I have a white, blue eyed, blonde grandson born to my dark and swarthy half Arab son, and his Eastern Europen wife who has jet black hair and porcelain coloured skin. My grandson could be a brother to Prince George, he looks nothing like his mum and dad - but he is the image of me. Do you see what Im trying to say to you apart from 'you are only doubting her paternity because you are angry'.
Your Granny - maybe she is just at an age where she can't cope with whats gone on, whats currently going on, and what could go on in the future. Maybe she just wants peace, maybe she had enough of it all when it was happening. Maybe she had your dad and his antics up to his ears and now thats he's dead she's thinking - I just want to live in peace.
I suggested to you that your dad caused all of your pain and heartache, that you had no more than she did because you both have the same cheating/weak/not very nice man as a father. And you then tell us about your dad taking you show jumping etc when much earlier on you spoke of how He would not include us in social gatherings and be aggressive and manipulative, hide us away a bit . It seems that if someone hits a nerve about your dad you come back and say 'ah but he did x y or z' even when it contradicts something you said earlier, and this conflict/confusion in the heart and mind of children who've faced what we have is common.
I have a feeling that after you're world fell apart your mum and dad lived under the same roof but nothing was ever resolved by them, that what had happened was always in the room and life was toxic. And I still think you need to stay away from your sister because of how hurt/damaged you are, but more than anything else I think you need to see a professional who can hopefully help you heel and move on life without feeling as if your shoes must be full of broken glass.
And You dont come across as arrogant to me because its obvious you desperately need to feel that you were the special one, the only daughter, and that you will grab at anything to help yourself feel this was the reality, even though its not.
You need way more help than I can give you, and all I can really do now is hope that one day you can come to terms with what happened and go on to live life with a peaceful heart and mind.