Convincing property owners to sell

Sure, a site plan is super useful to say "more" / "details" here. But as a "not done homework" to even be allowed to ask here, I wouldn't oversterilize that ;-) I think the OP clearly described the situation at the beginning: Mom lives in the house instead of a home, has already transferred the property to the daughters, but should still feel like the decision-maker, because the house even exists at all because of her. Mom is 80+, which probably means the daughters are already over 50, and they no longer have any stake in their personal living there. Not all heirs are vultures, some also love their parents. Therefore, I consider it very likely here that, first, Mom's word carries weight and, second, her vote may be emotionally influenced. Mom's world should not be shaken in her remaining days – including that her car has a familiar place. I am a businessman myself, but I can still take emotional stuff seriously. Always this whining from the business administration nerds when something beyond purely Excel facts is introduced into a discussion *LOL*

Maybe not with blunt statements, but possibly with emotions

In our family, a house is also being sold. If a family with many children makes the highest bid, then this family can buy it. If their bid is SLIGHTLY below the highest bidding investor, who needs to quickly invest their money, the family will still get it. If the offer is 10,000 below, too bad. The community of heirs is not a charity; grandma and grandpa worked their whole lives for it.
Why should it be made easy for strangers?

But shopping, mowing the lawn, sorry. The woman and her daughters are not stupid, they probably only said the thing about the mother’s decision and the garage so they could have peace.
And before grandma has to go to a nursing home and the house is lost because of it, a Polish caregiver comes into the house so that the property is preserved.

The daughters may be nice, but if they have children who also want something for the savings account, then it only works through the price.

Mom lives in the house instead of in a home, has already transferred the real estate to the daughters

I would be very surprised if Grandma had transferred anything.

If there was a Grandpa, I assume there was (unless it was the stork), then Grandma has 50 % after Grandpa's death and the two daughters each have 25 % and form a completely normal community of heirs.
When Grandma dies, both daughters form a new community of heirs with 50 % each.

And then it gets funny and really begins when there are partners who want to see money quickly.

Communities of heirs are supposedly the worst punishment for real estate agents.

From this perspective, it certainly makes sense to present oneself as good neighbors.

That is what one does, by the way! However, I also said that with these sentences, which the OP has quoted here, and which are presumably – at least in meaning, but probably also in wording – the opposite is achieved. That is sugarcoating. And it is not well thought out. It rather achieves the opposite, namely distrust.

but if someone offers me 200k, my emotions are also better than with 100k...

That’s right.

There is one point I haven’t said anything about yet: during Corona times and in general, it makes sense to introduce oneself in a letter. You have to show interest; otherwise, the other party does not have to put themselves in the situation that you might also sell it. However, it should rather be about the beautiful plot of land and for that, one should weigh very carefully the features mentioned in the letter. And yes, a realistic sales amount should be stated so that the owners can think about it for weeks.
However, there is a big difference between “not for free” and market value, at least that is the impression I have.
Hoping that the sisters/daughters of the 80-year-old do not already have children who could use the land for themselves, one can try it. And then provide a callback number and address. A kind of deadline like “it would be nice if you could think about it and let us know by the end of the quarter so that we can continue searching” would be worth considering.

Heir gemeinschaften are supposedly the worst punishment for real estate agents.

For agents who do not work properly. We had a house on the viewing list, so I contacted the agent on Saturday and on Sunday at breakfast came an email saying "viewings will be possible again soon." Since we were nearby anyway (and the house had a full address listed), we stopped by. A woman opened the door and told us that one member of the heir community alone had commissioned the agent without consulting the others. That also explained why the agent only had pictures from the outside... by the way, never heard anything from him again...

One kind of deadline "it would be nice if you could consider it and let us know by the end of the quarter so we can continue searching" should be considered.

Just not that. Then it is at the level of "business paperwork," which old people traditionally leave until they can discuss it with their children. But here it is about the message "dear Mrs. Grandma, here the nice young people are getting to know you, with whom you will gain new neighbors without anything dear to you changing." Grandma’s pro-vote is the door opener to the daughters. That Grandma is the "trusted preliminary decision-maker" for the sellers should be respected and not reinterpreted. : google Berliner Testament. Grandma may have effectively arranged that the real estate already belongs to the daughters.