So sorry or your loss, i totally understand your position. I do tend my fathers grave, but not as much as it should and we haven't got him a gravestone yet because we can't afford it. Im an only child so no one to share the cost with. My DP made a wooden cross but it needs replacing.
I don't feel guilty though because i don't need to show other people that i loved my dad. I also know that he wouldn't want me to spend money that i can't afford on something like a headstone, he also wouldn't want me feeling dutybound to visit his grave. I think about him every single day, don't need to visit the cemetary to do that. When i go, i go for me, not for anyone else.
My DD1, has never visited her grandads grave, it upsets her too much, so she doesn't go - I totally respect that decision.
If it is that important to the rest of your family, then get them to help out financially, you can pay for the graves to be maintained, so tidied, cleaned and sometimes flowers planted (depending on what option you go for) its usually a one off payment for say, 50 years. If everyone chipped in, it may be affordable? Your relative sounds like a bit of a cow if im honest