Plumbing Questions? Ask a Plumbing Expert.
Do you have adequate hot water in the showers? particularly the one with the problem sink? how many handles does the problem sink faucet have? 1? 2?
What do you have for a water heater? brand and model please or whatever info you can provide if you're not on site. I'm logging off for the night soon. I'll respond in the morning if you don't get back to me in time
Your best bet is probably to call a plumber unless you are comfortable taking the faucets and valves apart. If you haven't already you could try turning up the temp on the water heater. It's possible that the plumber who installed it was being conservative and set it too low for your needs. This thread won't go away and you can come back here any time. I'll even answer related questions after you rate me favorably.
If you're ok with taking things apart let me know and I'll give you some options