Hello Birdlady and welcome to Pond Boss. Personally my advice would be to talk to an attorney and soon. I don't know whether or not you would want to sue family members but even if you don't you should find out what your rights are. Regulations regarding building dam and changing the flow of ground water varies from state to state.

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)