Theoretically speaking he could have spared the boys. From his perspective.
It wouldn't have effected his inheritance as they were children and would have gone to Colin in the interim before being allowed to stake a claim for Sheila's share. Which would have probably happened in 97 when they turned 18.
12 years would have passed by then.
He was jealous of them. He told Liz Rimington what "
spoiled brats" they were. The contrast to how emotionless he had been raised to the affection lavished on them by his parents was too much for his fragile ego to bear, already damaged by a sense of failure at Gresham's.
As far as the inheritance went it would complicate things for the future as you point out, but he also wanted use of the Maida Vale flat for his courting exploits. He scornfully packed Nicholas and Daniel's possessions into bin bags for Colin to collect, all of it merely refuse in his mind, the detritus necessary to clear before he could finally begin to lead the life he chose.