I'm actually in a similar boat. I raised my niece who is about to turn 14 in 2 weeks since she was 3 years old. Her mother, my cousin was a party animal then and always left my niece with my grandmother who lived with us. she doesn't have a father that is active in her life either so im like her big brother, friend, fatherish figure. She also has a half sister now that is turning 5 soon, who i raised for for a year. My nieces are a huge part of my life and how i use it is use my experience of being playful with my nieces on girls.
I used to trick my niece into giving her kisses on the cheek by telling her i had a secret to tell her, or that there was a bug or something on her face or shoulder, i even convinced her one time i could hypnotize her and i took a pencil and started swaying it back and forth (totally did not think it would work on her but it did), and i use these same things on females and they love it. As to your social circle, that's just something you have to slowly build up, I wouldn't mention raising your nieces or anything as it doesn't really help you much. If anything people find it weird that you love little girls (its happened to me before. My father used to think i might have an issue as well but now they understand she's just really important to me).
little girls are really not that different from older girls when you think about it. Spoiling a little girl will just make her walk all over you, instead you gotta be the big bad older figure and nice to her sometimes. Work on expanding your circle and also start an Approach Journal.
Approaching more women outside of your circle will increase your social circle as well. Once again, i'd refrain from mentioning more than a sentence or two about your nieces, they may mean the world to you but its probably something you should keep to yourself as not many people will understand that.