Why does the water at my sink not get hot but the shower does (same bathroom)?
Does your sink faucet have a single knob that controls temperature and pressure, or does it have separate knobs for hot an cold? If it's the single type it probably has some corrosion in the valve that's letting cold water in with the hot. If you've got shutoff valves for the sink faucet, turn the cold water supply off and see if it still only gets lukewarm or not.

Depending on brand and age of the faucet, you should be able to get repair parts from your local home improvement or hardware store.