"If you think a hundred pounds a month is too much, then I agree with others - you are definitely lying about supporting the mum’s mortgage"
The OP is in Scotland. In some areas houses cost from £30k. I'm in Liverpool, my DD has just bought a house, her Mortgage is £320 a month, so half of that would be £160. Her friends Mortgages, bought 10 years ago are under £250 a month. We don't all live around London.
It isn't jealousy that means you don't want to give money to another Adult, who has more coming in than you.
I'm in the position that two of my DDs earn more than I do, so do my Grandchildren's other Grandparents. You can't bankrupt yourself trying to keep up.
I agree with speaking to the ex to see if anything is needed.
The DD is taking the piss out of both of them, by the sounds of it.
As for the OP being on low money. We need a level of people to do lower paid jobs in our Society, otherwise we wouldn't function.