My husband has been receiving his full SS benefits for some years now. His disabled daughter is also getting payments under his benefits. I turn 66 in March 2019 and my plan is to apply for benefits under my husband’s until I turn 70 when I would then apply for my own benefits. My question is whether I receive decreased benefits under my husband’s, since his disabled daughter is already receiving benefits through him?
I can't say for sure, but it sounds likely that your spousal benefit rate would be lowered due to the family maximum benefit (FMB). And, your drawing spousal benefits would also likely lower your husband's daughter's benefit rate for the same reason (https://www.ssa.gov/OP_Home%2Fhandbook/handbook.07/handbook-0732.html).
Our maximization software can handle the calculations involved for FMB cases, so you may want to consider using it to determine your options with regard to benefits.