What is known about local structure of actions of semisimple groups? More precisely, suppose I have a semisimple group $G$ acting on a variety $V$, and $x\in V$. Assume that the stabilizer of $x$ is a parabolic subgroup $P\subset G$. Can I always find a slice $x\in V'\subset V$ such that the natural map $G\times_P V'\to V$ is a local isomorphism near $x$?

[Edit]: my previous counterexample was irredeemably wrong; hopefully this one works. Suppose that there exists an étale $G$equivariant map $V' \times^{P}G \to V$; then there exists an invariant neighborhood $U$ of $x$ in $V$ (the image of this map) such that the connected component of the identity in the stabilizer of any point of $U$ is conjugate to a subgroup of $P$. Let $G = \mathrm{GL}_3$ acting in the natural way on the space $V$ consisting of pairs $(p, L)$, where $p$ is a point of $\mathbb P^2$ and $L$ is a line in $\mathbb P^2$. There are two orbits, one consisting of pair $(p, L)$ such that $p \in L$, call it $I$, and its complement $V \smallsetminus I$. The stabilizer of a point $x$ of $I$ is a Borel subgroup $P$ of $G$. However, the stabilizer $H$ of any point of $V \smallsetminus I$ is connected, and is not contained in any conjugate of $P$. In fact, it $H$ was conjugate to a subgroup of $P$, it would stabilize a point of $I$; however, it is easy to see that the only fixed point of $V$ under the action of $H$ is $x$ itself, and $x \notin I$. So in this case there cannot be an étale slice. 

