preschooler criticizes little brother

Any tips on handling this? My dd is 4 this month, and my ds will turn 2. DD often finds fault with the way ds plays -- either he's not doing it "right" or he is playing a "wrong" role. I know this is totally developmentally normal for 4, but I could use some practical tips on how I respect that while protecting ds's imaginative play. Example: This evening ds sat down to "nurse" a big stuffed bear which he called his baby. (It's the first time he's done this -- so sweet!! ) DD moved to rip the bear from his lap (I stopped her), stating loudly, "Your a BOY! You can't nurse! *I'm* the mommy!" I told her he could pretend, and that I wanted her to stop yelling at him. "NO! He's a BOY!" Etc, etc. After a minute, she gave up on the gender role angle and started in criticizing his technique in a bossy tone of voice. (He had the bear tummy down on his lap rather than facing his chest.) "THAT'S not how you do it!" Etc, etc. I finally had to threaten to physically remove her from the room. She stomped off, still grumbling. UGH!

Again, I know this is normal -- she's in the phase where she knows how things are *supposed* to be, and feels strongly about having things happen that way. But I feel bad for ds! I try to validate her feelings but restrict or redirect her behavior. However I can't shut her up, and I feel her criticism of him is the worst part! Any tips, or do I just suffer through this phase? They do play together pretty well most of the time, and I have to admit that ds doesn't seem too discouraged by her words. He only squaks when she tries to make him stop doing something or do something different. Thanks.