This is such an interesting ask because Ryuken and Isshins personalities were largely altered by the death of Masaki. Ryuken regressed inwardly and grew colder to those around him while Isshins extroverted personality caused him to be even more affectionate and outgoing in order to shield his children from the grief of losing their mother.
I think the relationship would start off as a tale of unrequited love, at least in Ryukens eyes as he watched the person he hates the most move on from Masakis death with someone new. Someone he himself had feelings for.
The key to winning him over would be their refusal to let go.To continue being the balm to his old wounds and slowly starting to integrate him into their life more and more until his feelings have settled enough to have the big conversation.
The idea of a poly relationship with Isshin would repulse him initially, but at this point the pair have become an absolute pillar of stability in his life that cutting them out would feel like self sabotage.
He needs them. He needs them with him.
And once he finally admits it, everything else falls into place.
For the first time in nearly a decade he feels happy. Content again with his blended family.