Travel insurance refused my claim for a cancelled trip

It depends on the policy wording. Many travel policies exclude pre-existing conditions unless they are declared and accepted by the insurer, and the definition can be quite technical, including whether the condition was stable or whether treatment had occurred before the policy was purchased. The insurer must show that an exclusion applies and must apply it as written, without stretching the wording. Check the definition of a pre-existing condition and whether it applies to family members who are not travelling, as cover for a relative's illness is often limited. Ask for the decision in writing with reference to the exact clause and the medical evidence relied on. You can complain to the insurer and then the dispute resolution scheme. Meanwhile, ask airlines and accommodation providers for refunds or credits.