What should I do if there is a pre-existing medical condition for a dependant that might affect eligibility for immigration?

Its recommended that you have an understanding of the eligibility for yourself and your dependants, including a for temporary residence, and an understanding of likely eligibility for permanent residency.
We recommend that you contact an immigration solicitor with experience in such matters, such as https://www.linkedin.com/in/munashe-rusamo-1281b120a/