I hadn't considered it rent, but in a similar situation to you, we have Mil living with us. I had to 'give up' my home office to accomodate her - and also give up a lot of the free lance work I was doing, in order to provide her care. She hands over her attendance allowance, plus a small weekly sum - that money covers all her food, toiletries, the hand gels we go through like water, the cost of replacing her 'pop socks' and not infrequent purchases of underwear, her contribution towards our bills (which have gone up a lot due to higher heating costs and especially because of extra washing due to incontinence) and - a really big expense - petrol costs, as we pick her up from day care 5 days a week, also frequently have to take her in, run her to visit friends and to hospital/doctors appointments - usually its about an extra £45 pw going into the car alone, plus the fact that (like today) youngest and I are off to visit oldest in Manchester, and not being able to use the car as OH needs it to pick his Mum up, its costing us £30 in train tickets (£10 roughly more than the petrol costs for a straight run in the car) - I've found that there are a lot of similar additional expenses like that. We're still slightly worse off than before she moved in, mainly down to my income being reduced a lot - whereas Mils 'costs', compared to when she was living independently, have reduced by about £300 pm.
Although Mil has now declined to the point where she is thinking in terms of 'shilling and pence' and feels that I am something of a spendthrift for paying £1.50 for a lip salve when as far as she is concerned, they can be bought for about sixpence, she did agree to these figures when she moved in - intially, I wasn't happy about the amount, worried that it seemed a lot, but when OH sat me down with pen and paper and we looked at the extra costs, it was clear that Mil was a lot better off financially - and we weren't, even without my loss of earnings coming into play.
So no - I don't think you are being unfair or unreasonable either