I am disabled on benefit’s , I have 1 child i fully support and do not receive child support , I am currently fighting my ex for this a back owed support for my son up to 6 years .( this is another story )

My son and his girlfriend live with us , my son earns over £30 k a year and his partner is also disabled like me and receives the same money as me . Up to now I have only asked for £340 a month towards every thing a month from them . I am seriously financially struggling now , I have zero savings and cannot afford any thing for me at all , I’m struggling to provide for my younger child . Last time I brought up increasing rent he was furious and I feel unfair . I’ve worked out that if my son and his girlfriend moved out it would cost them well over £2000 a month to live , also his girlfriend has a food allergy which means food is more expensive , in fact the average for a couple in my area a month , just for food is £569. A month . Last time I mentioned this he made me feel I was being unfair for asking . I feel I should be asking for a lot more for the two of them for rent , and am being taken advantage off . Also neither do any housework , neither help me in any way around the house, garden etc and I feel exhausted and fed up Suggestions for rent a month and how to handle lack of help . Do you think I’m being unfair ?