Hi Barbara,
Its useful to get accurate weights if you can as it helps to make the right decision. Its sounds as though they might both be borderline weights for survival based on what you have. Certainly its possible though for even tiny ones to survive in the wild if they have an environment that supports their needs. taking them in is very stressful – for you as well!
The decision to overwinter is a significant one – and I would always be inclined to leave well alone if you know they are as healthy as they can be and that you are prepared to support them with a good food and water source through the winter.
As you say – there may well be more of them – and I seem to recall having read somewhere that small siblings stand a better chance of survival when left together under such circumstances.
The control of temperature you mention is really for indoor recuperation purposes – they typically recommend above average of 18deg C to reduce the risk of them hibernating and improve chances of them making a recovery if they have been ill. Once hogs are healthy and make a good weight they can be moved to cooler environment to encourage hibernation before release in the spring.
Not sure if that helps your dilemma – but worth getting them a little check each from the wildlife vet or carer to help the decision.
