Firstly I hope her name isn't Sky because that's what you wrote in your OP. I found it very hard to understand your posts as it jumps from her DSon to yours to her DD to something about clothes which I just simply don't understand as it seems all over the place and I can't work out who has what DC. But, what you said doesn't sound like she's being mean. Maybe she has ASD, or is just shy/socially awkward so gives presents to people she knows? You haven't answered a PP's question as to how much effort you have made to get to know her? Maybe she feels that you have been snubbing her.
Why does your DFiance (not DP anymore, since you're engaged!) have to convert, especially if you're only having a Registry Office wedding, and not even a religious one? When you marry someone, you take them as they are. Why can't you accept him as he is, why do you need to change him and his beliefs to marry him? Why should he have to change, just to marry you? Why don't you change instead? Better still, why can't you be Jewish and he be...... whatever that is? It is seems unfair, false and hypocritical.
How can his sister take up 4 places? If you can only invite 10, you invite 10 - them themselves, no partners/children. None of this makes sense. If you're limited to 10, common sense would suggest you would invite 10 DIFFERENT PEOPLE. Not 4 people with one person. I'd say his mum, dad, sister, brother. Your mum, your dad, best friend, sister. Add a friend (best man) for him, that gives you 9. And you've got a spare left for a friend or relative. I really don't understand what the issue/s are. She seems shy and reserved but nice, you're not even having a Jewish wedding, and as I showed, you can invite her and a couple of friends, PLUS both sets of parents. How can this possibly be more straight forward?