It sounds sad to me, not weird. I would think it is a decision linked to the hoarding - the impossibility of letting something, or rather someone, go. It is perhaps the only way they can cope with their mum's death, to have her so near, and it will hopefully bring them some comfort. We don't all grieve, or view death, in the same way. They're not harming anyone, and there are far worse neighbours to have.
The family near to us who also did this had a large garden but not quite a country estate. There were no MH or hoarding issues. They just wanted their family members to be in a place they had loved over many years, and to be close by. When the house was sold some years later following the death of the surviving parent, I understand that a codicil was put on the deeds stipulating that the graves could not be disturbed. It was a lovely house and sold quickly at a fair price.
It wouldn't bother me, and I'd have no objection to letting family members visit on anniversaries once or twice a year, or even putting flowers on or keeping it tidy myself. Having lost both parents, I know how important these things can be. We all need a bit of care and kindness, after all - including these brothers, by the sound of it.