I'm going to provide some D-1, D-1(B) and D-3 charts here and try to figure out the number of siblings using the points I mentioned before. I'll go through as many horoscopes as I can in the order that I come across on my computer and do the analysis. Along with each example, I'll also mention whether it tallies with the real data or not.
D-1: Ju,As,Ke: Sc; Ra: Ta; Mo:Ge; Me,Ve,Ma: Le; Su: Vi; Sa: Li
D-1(B): 1:Ju; 2:Ke; 8:Ra,Mo; 10:Ve,Ma; 11: Me,Su; 12: Sa
D-3: As: Sc; Ra,Su: Cp; Sa: Aq; Ju: Pi; Me: Ar; Ke: Cn; Ma,Ve: Le; Mo: Li
D-1: There is no planet in debilitation. Sa is exalted.
D-1(B): L3 exalted in H12 with no malefic aspect. L11 is in H11 along with a friendly planet with no malefic aspect. Except that Mo is in H8, there is no hint that the native is the eldest/youngest born. Since the native is a girl, the rule about Mo can't be applied to her.
D-3: As is in even sign. Again no hint here to claim that the native is eldest/youngest sibling. So, count younger siblings backwards.
First one corresponds to Vi. It's a feminine sign owned by Me who is feminine. Me is in 8th sign from Vi. But, Ju is aspecting the sign from Pi, strongly. So, the issue may survive. It has to be female.
The next one belongs to Cn. Ke is present with the aspect of Su. Note that Su is not such a strong malefic and Ke has the aspect of Ju again. Ke can give both male and female issues. Since the lord of Cn, Cn itself are feminine, I would predict a female issue.
The next issue belongs to Ta. Lord of Ta (Ve) has malefic aspect of both Ma and Sa and is in an inimical sign. So, there may not be a next issue or there may be a loss.
Moving to elder siblings, the first one corresponds to Cp. Ra, Su are there. Both of them are masculine and so is Cp. So, this must be a male issue. Su is a bitter enemy of Sa and Ra is a malefic. So, there may be a loss for this one. Note that Mo is in H10 for this issue and Ju is in 11th sign (remember, you got to count in the reverse order). So, this issue may be the only elder issue for the native. I'm stopping here.
Prediction: Elder brother, 2 Younger sisters.
Fact: Has two younger sisters and one elder brother. I'm not sure of any losses. Will find out and update.
D-1: As, (Ma), (Ju), Ra: Le; (Sa): Vi; Me,Su: Cp; Ve,Mo,Ke: Aq
D-1(B): 1: Ra; 2:(Ju),(Ma),(Sa); 6:Me,Su; 7:Ve,Mo,Ke
D-3: As,Ra: Le; (Sa): Vi; (Ju): Sg; Me,Su: Cp; Ke: Aq; (Ma): Ar; Ve,Mo: Ge
D-1: No debilitations and exaltations.
D-1(B): There is no indication that the native is the eldest/youngest in the horoscope.
D-3: As is in odd sign. So, we count the siblings clockwise. Again, there is no indication that the native is the eldest. However, L3 in H11 indicates that the native is the last born for his parents.
H11 has Ve, Mo and (Ju), (Sa) aspecting strongly. It's a masculine sign. The lord is feminine and is in 8th sign from it associated with a masculine planet. Ju/Ve 7x7 in Ge makes sure the issue survives. It's not clear to me as to whether this issue is male or female. With my limited knowledge, I would predict "girl". For this particular issue, there is no indication that this is the eldest one.
The next issue corresponds to Ar. This is undoubtedly male because, the sign, the lord of the sign, the planets located and aspecting the sign are all masculine. For this sign, there is an indication this is the eldest son. Since, the L11 is in 8th sign from it with no benefic aspect. And H11 has Ke in it with no benefic aspect.
Prediction: Elder brother, Elder sister, No younger siblings.
Fact: Elder brother, Elder brother, No younger siblings.
D-1: As, Mo: Sg; Ke,Ju: Ar; Me,Su,Ve: Ta; Sa,Ma: Cn; Ra: Li
D-1(B): 1: As,Mo; 5: Ke; 6:Ju,Me; 7: Ve,Su; 8: Sa,Ma; 11: Ra
D-3: As: Ar; Me: Ta; Sa: Cn; Ke: Le; Ju,Mo: Sg; Ve,Su: Cp; Ra: Aq; Ma: Pi
D-1: Ma is debilitated in H8. No direct relation to H3 or H11. There is cancellation of debilitation. Mo in H1 may indicate that the native is the eldest son.
D-1(B): There is nothing important.
D-3: As is in odd sign. So, we have to count younger siblings in clockwise order.
First younger sibling corresponds to Ge, masculine sign with the lord being feminine and in a feminine sign. It is being aspected by a strong Ju, Ma and Mo. Considering Me as dual planet, my prediction is more inclined towards a male issue.
The next issue corresponds to Le. It's a masculine sign and the lord is also masculine. Su is in H6 from this sign along with a strong female planet in a feminine sign. Note that Su is not strong in an inimical sign. But he has the benefic (and feminine) influence of Ve. A strong Ju also aspects Le. It's beyond doubt that the issue would survive. I would predict a male issue for this one although there is some ambiguity. I don't see a strong reason for why there should be no younger issue from here on. However, a reasonable signal for not having younger sibling here is: L3 from Le is in H6 with no benefic aspect, but the malefic aspect of Sa and Su.
Coming to elder brothers, the first sibling belongs to Aq. Lord of Aq is in H6, weak and with benefic aspect of strong Ve. A strong Ra is also placed in Aq. So, there may be an elder brother. There are a few other "weak" reasons to predict that this issue is improbable. But I don't want to consider them to overfit our model. Let's stop here.
Prediction: Elder brother, Younger brother, Younger brother
Fact: No elder siblings, Younger brother, Younger sister
So far, one thing is clear: Wherever there is a mercurial presence, there is a confusion as to whether the issue is male or female. It should be noted that the D-1(B) chart of Ex-2 is very similar to the 2nd younger sibling of Ex-3 in D-3. And in reality, they are brother and sister. This gives me sufficient energy for me to pursue further research in this direction.