A paper of Bezrukavnikov (http://arxiv.org/abs/math/0604445) identifies this as $\mathcal{O}(-1) \oplus \mathcal{O}(-1)$ in Example 2.8. For only somewhat related heuristic reasons (which I can go into if anyone cares) I believe this to be true. Here is a partial argument I could come up with.
The identification of $G$-equivariant vector bundles on $G/B$ with $B$-representations gives us a short exact sequence of $B$-representations: $$0 \rightarrow V_{-2} \rightarrow \mathfrak{b} \rightarrow V_0 \rightarrow 0$$ which gives us a short exact sequence in vector bundles: $$0 \rightarrow \mathcal{O}(-2) \rightarrow \tilde{\mathfrak{sl}_2} \rightarrow \mathcal{O}(0) \rightarrow 0$$ My guess is that there is no decomposition theorem of $SL_2$-equivariant vector bundles on $\mathbb{P}^1$, so this is as good as it gets in that case. If we are willing to pullback to the non-equivariant case, then we get a short exact sequence: $$0 \rightarrow \mathcal{O}(-2) \rightarrow \mathcal{O}(-1) \oplus \mathcal{O}(-1) \rightarrow \mathcal{O}(0) \rightarrow 0$$ which can be written down explicitly: the generating sections on one affine cover should be $e, h \in \mathfrak{sl}_2^*$ and $h, f \in \mathfrak{sl}_2^*$ (respectively!) on the opposite affine cover (here I am thinking of $\mathcal{O}_{\tilde{\mathfrak{sl}_2}}$ over $\mathbb{C}[\mathfrak{sl}_2]$, so I really mean whatever their images are in the map on rings; the $\mathbb{C}^*$-action on the fibers may also help in thinking of those elements as being in "degree one").
It was helpful for me to think of this as a "twist" of the usual short exact sequence for the tautological bundle on $\mathbb{P}^1$: $$0 \rightarrow \mathcal{O}(-1) \rightarrow \mathcal{O}(0) \oplus \mathcal{O}(0) \rightarrow \mathcal{O}(1) \rightarrow 0$$ though I do not know a "geometric" reason why these two should be related (if there is a reason at all). I would very much appreciate it if someone could give a better explanation, or correct me if I am wrong.