YA or adult, 1990s, middle-class guy dates classy rich girl

This was probably YA, but might have been an adult book. I was lent this book by a friend of my mother's when I was probably too young to be reading it, so it stuck with me. I read this probably 1995-1998, and it seemed modern. I remember next to nothing about it, so my Google-fu is useless and it probably needs to be identified by someone who has read it and recognizes it.

It's about a middle-class guy, probably late teens, who somehow starts dating a classy rich girl who lives in a mansion with servants. He lives in a small cookie-cutter house with his family, and was in awe of how much land her house was on. I remember he has a little brother, I think named Sky or Skylar.

I remember that the hot rich girl was described as "slim yet full-figured," because I had to ask my mom what that meant.

I remember the end of the book there was a costume party at the rich girl's mansion wherer they were out in the huge yard. The theme of the costume party was to dress like what you were going to be like in 10 (or some number) years. The rich girl put a pillow under her shirt like she was pregnant and the guy asked her if the baby was going to be his, and she says no. I also remember that her family's elderly butler was wearing a halo and wings, because he expected to be dead in ten years.

I want to find this because my mom's friend must have seen some value in it that she wanted to impart to me, but I was too young to recognize it.

Bump.

Someone solved this on Goodreads! The answer was Gentlehands by M.E. Kerr.

Thanks much for letting us know! I'm glad you were able to find it.