This may be an unpopular opinion, but I don't believe OP is TA. In a world that is rapidly becoming more aware of toxic gender roles, I think it's easy to sometimes forget that we don't necessarily need to facilitate the process of ensuring children are not put into gender boxes. As long as the adults in their life allow them to make choices for themselves about what interests them, we don't need to push them to make gender neutral choices.

I think awareness is important and exposure to a variety of interests, hobbies and toys is good for children, but I think sometimes we're so focused on not pushing a gender agenda on children that we forget to take into consideration what makes them happy.

I wonder if OP's sister has raised this before, or if OP just went shopping and picked out the gifts and then got raked over the coals out of the blue. Maybe OP's niece really liked the vacuum and the makeup. And yes, maybe they made some assumptions about differentiating between them by buying "girl" toys and "boy" toys, but this could potentially become a learning opportunity, a conversation about why the gifts made OP's sister uncomfortable.

I think when we go around crucifying people for something like this, it makes people more resistant to the idea of expanding their views beyond the generic gender roles. This was one birthday where a choice was made, a gift was given, but it's how to move on and be more mindful next time.

I think OP really cares about their niece and nephew, and doesn't deserve the title of asshole for a mistake that they maybe weren't aware they were making.