Remember, you have to get multiple people to agree - and their motivation levels probably differ dramatically. I hate being the listing agent in these situations, it is very difficult.
Also, remember that 1974 means nothing - find good comps.
I believe you have answered your own question. Have your Realtor write an offer 25% below the asking price, since you don't want to be too aggressive. Then you can readily gauge the motivation of the sellers or at least one or two of the sisters. You will know the true market once both sides fully agree to the sales price.
Happy Good Friday,
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