Bath Faucet Water Issue

[TJ1975]

I have a relative selling a family home, and I have been helping out with the remodel (though not with the actual work). A contractor, who is no longer on the job, did most phases of the reno, and an issue has come to light with plumbing in the bath rooms. Both baths have hot water in the showers/bath tub, but the water in the sinks does not get hot. If, and only if, the showers are running, the water coming out of the sinks will get luke warm, but never hot. Turn off the shower and water is not even warm. And you can turn on one shower, and warm water will come out of both sinks (one in the other bathroom also). We had a few plumbers look at it, one wasn't sure of the problem, another thought it was a mixing valve issue. My relative, who is fairly handy, replaced the mixing valves, but that did not help. Now someone has suggested it might have something to do with there not being enough pressure to get hot water to both showers and sinks, but not sure if that makes sense. If that were the issue, wouldn't the sink water be hot when the shower was turned off? When you turn on just the sink faucet, water is neither warm nor hot. Turn on either shower, and sink water becomes warm only. Perhaps it is a pressure issue after all, and there isn't enough to draw hot water to sink. If so how would that be corrected? Does anyone have any idea as to what the issue might be, or how we can resolve it? It's got us stumped. Many thanks for any tips or advice!