Kids don't even "play" together until they're closer to 3 years of age. Even then they can still do a lot of parallel play. Playing similar things side by side but not together.
With her being over age 3 and him being under 3 your expectation of them playing together, with the same toys, concerns me.
Surely you aren't making your pre-school child play with baby toys to appease her younger brother and you aren't allowing a toddler to play with toys specifically meant for kids over the age of 3? Are you keeping their toys separate? He should never ever have access to her toys. Even Barbie shoes can be a choke hazard. Little wheels on Hot Wheels or other toys like them have dangerous parts than can break off and choke your child.
She should be playing with educational toys and have a wonderful pretend play area with dolls, dress up clothes, dishes, a play kitchen, hats, purses, shoes, and more: 4-10 piece puzzles, art with paint and crayons and markers, blocks, have tons of books to board books to read and some paper books if she's good with them, and she should be able to have a sensory table or something similar...outside is good for this sort of thing too.
He should have toys that have NO pieces that are smaller than a toilet paper cardboard roll. Nothing at all that would fit between his teeth if his mouth was fully open. Even a big wheel can come off and be put in the mouth.
So I am wondering about what you mean when you say play together. With their age difference I can't imagine her wanting to play with his things.
I kept the kids toys in their own room. If they came out of their room the toy got put in time out for the rest of the day. Our girl had a gate in her doorway so that her younger brother wouldn't be able to go in her room. If she wanted her door shut we would allow that too. We had a baby monitor on top of the shelf in the hallway so we could hear anything going on in both kids bedrooms.
I know you want your kids to be close, that's not what I addressed with my comments. Trying to make them play together at this age isn't going to be something they are cognitively ready for. You can google side by side play and parallel play to see how that works and what it is a stepping stone for.
As for them being close, spending time together as a family doing fun activities they both enjoy is a good way to foster that love. Being a role model to her when you speak to them is the best way, in my opinion. If she is constantly hearing "He's a brat" or "He kept me up all night again last time and I'm exhausted" then you can't play with her because you're too tired...that makes her mad at him and then she starts to resent him. So speak positive things about both kids in front of the other one. Show that they each have strengths and they are separate, where they are still family but have independent strengths.
Overall I would think by the time he's 3.5 they'll be a lot closer.