I know I can give cash gifts to my grandchildren tax free, but can I also make such gifts to my sons and daughters-in-law?

Gifts are not deductible for income tax purposes, but may be exempt from gift taxes. For 2019 you can give up to $15,000 per recipient (same as for 2018) without being subject to gift tax. There is no limit on the number of people to whom gifts eligible for the up-to $15,000 exclusion can be made. Thus, if you have two sons and two daughters-in-law, plus five grandchildren, you can gift tax free up to $135,000 ($15,000 x 9 gift recipients). And if you are married and your spouse consents to the gift, you can give double this amount from your property.