I nursed my sick husband for years, yet my share of the estate is the same as my co-wife's, who rarely came. Is that fair?

Your sacrifice is real and not wasted in Allah's sight, even though faraid divides equally between wives. Faraid fixes rights by status, not by service. Your service is recorded as a great act of worship, and in worldly matters there are still other channels. Your financial contribution or effort towards property built up during the marriage can be claimed as jointly acquired property (harta sepencarian) before faraid. If you paid for your husband's treatment intending it as a loan, discuss the possibility of claiming it as a debt from the estate. Get a syarie lawyer's advice on both matters. Pour out your disappointment to Allah and to people you trust, not to your co-wife at the condolence gathering. See matrimonial property (harta sepencarian).