It's unfortunate how the past memories can cloud what good memories we can make now. Just think, now (I assume) you and your sister are old enough to have genuine conversations, to become actual friends instead of just a brother and a sister. Independence does change people and make them grow, and you will probably go through that as well. It's just important to realize that a) those memories are mystified of simpler times and b) she probably feels similar about those times. Don't think you took those time for granted, you didn't have the mental complex to appreciate those times. Now that you do, make the effort to have a genuine connection with your sister as mature individuals.
You can't go back to being a bunch of kids playing in the grass, but you can reminisce alongside her and build new memories as new people. I genuinely put time, effort, and thought into this.
Qualifications: 18 year old done first year uni with a high school brother. I have thought these things before hand just randomly.