In fact, I suggested that if the mother is worried she should talk to the pediatrician simply because I thought the pedi would reassure her that this is normal. lol
Oh heck, my son went through a stage like this and it lasted a while. He loved all things pretty and played dress up and got my things to do it, etc. By 5 or 6, he was completely out of it. He's now 7 and is into baseball and star wars. He had a barbie at about 3 and my 'manly' husband would just roll his eyes as he would try to take that to the hardware store with him. He never said a word (my husband that is) and our son and he would head off with barbie in hand. Again, by school age this was over.
Many kids love their mommies and love mommie's stuff.
I don't know about talking to a pediatrician because it is not a disorder to like feminine things. That is just the way he is. He may grow out of this and probably will or he may not. You can't make someone different than how they are. I'd not make a big deal out of it and just play all things with him. Get him out there doing some guy things too such as riding his tryke, kicking a soccer ball, get him a super hero. Introduce him to Thomas the train if you haven't already . . . get a dvd and a train for him and start a whole love of trains (very boy like).
So no. Don't be concerned. Either way. If he grows out of it, great. If not, that is who he is and what are you gonna do--------- make him feel terrible for it? But my bet is he'll grow out of it. good luck
What can you in fact do about it? He is not your child.
Realistically, he is only 3. If you said he was 10 and all this was true, I would say that some of the stuff I've read from gay and transgendered men says things like "I knew from the time I was 10" and such, but nothing I've read suggested a strong desire to cross-dress for sexual-identity reasons that began so young as 3.
If I had to guess, I would say that someone he cares about talks to him and is impressive to him, and that person role-models girly things, like high heels, and puts a big emphasis on "pretty" stuff.
If he were my boy, I would try to ignore it at this point, and would definitely encourage his stepdad to do the same. If your sister is worried about it, she could consult his pediatrician.