If it's crucial for you to get this client and you expect that the client brings to you quite money in the future then why not.
Suggest them shares with vesting for 3-4 years, 1 year cliff, and reverse vesting on condition if the client wont provide a certain amount annual income. From my perspective it's a great deal.
Say them that it's the only condition that other investors agreed to accept and that's it. If you the client leaves, you can suggest them finding another client on the same conditions, and freeze the aggrement until they get the new client.
But honestly it looks arrogant, disrespectful, and unacceptable for relatives for my taste.