The sink in the bathroom upstairs has a hot water as expected. But the shower/tab faucet does not and this is the case only in winters. Even if I run it long enough, it gets to slightly luke warm at best. Funny thing is in summers it is fine. Everything is fine downstairs and the heater is in the basement.

Please advise.

The house: 12 years old. Gas water heater I assume but not sure. I see gas pipe going into it. It was replaced 3 years back by previous owner.

Is the shower valve a single valve, or double. I am guessing single. In that case, the mixing valve has a limiter on it that only allows it to be turned up so far. I suspect in the summer when it is hot out, the water gets to the shower hotter (ambient temp in the house is most likely warmer, so it does not cool off as much on the trip to your bathroom), and you also perceive it to be hotter as your skin is not as cold. In my opinion, the solution is to remove the plate covering the mixing valve and adjust the limiter to allow more hot water. I can't go much further as I don't know the details of your valve. An online search will probably give you the details on how to adjust it.

I think you have confirmed my hunch. I will try that, if I get stuck, I will reply with a pic of the valve... thanks for the help so far..