I'm sorry to hear you're having a rough time quilting this one. I know it's doubly special since it's for your sister. Maybe before you get too far into it, you might take out the quilting you've done and assess what needs to be done to keep it from bunching. Then start again. Maybe more pins or basting are needed to get a smoother finish. I started over twice before I finally got the last quilt to mind! It was a pain taking it out, but I'm pleased I did. If it's just minor puckering, I usually leave it alone and just hope the recipient doesn't look too closely for mistakes. Luckily, most of my quilts go to people who don't even sew, so the mistakes that glare at me are easily forgiven by them :) Good luck. You do such nice work I know you can make this quilt behave!