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The issue here is about capital gains tax.
There really is no way to minimize capital gains tax.
Whether the mother gifts the shares to the son and daughter-in-law or sells the shares to the son and daughter-in-law or whether the shares pass to her estate upon her death and then to her son and daughter-in-law, the capital gains tax would be the same
There is a deemed disposition at fair market value no matter how the shares are transferred to the son and daughter-in-law or to the estate. At the same time the son daughter-in-law will be deemed to have acquired the shares at fair market value.
So the mother will pay capital gains tax or theestate will pay capital gains tax if the shares are worth more when she transfers them than when she inquired them.
If the shares are held jointly or are gifted or sold to the son and daughter-in-law before the mother's death then this is a way of avoiding probate fees.
Let me know if you need any further clarification.