Maybe it is, but it's not thicker than paint - portraits of siblings

 

Siblings, by George Romney, circa 1772-73.

Five children of the Budd family, unknown artist, circa 1818.

Maximilian, Sophie, and Ernst, the children of Archduke Franz Ferdinand and Sophie, Duchess of Hohenberg, unknown artist, 1908.

John Oscar Kent and Sarah Eliza Kent, by S. L. Gerry, 1844.

Master John Truman Villebois and his brother Henry Villebois, by Thomas Gainsborough, 1783.

 Siblings, by Harrington Mann, 1926.

Magdalena and Jan-Baptist de Vos, children of the artist, by Cornelis de Vos, circa 1622.

British school, circa 1810-25.

Siblings with goldfinch and chicks, by J. M. Combelle, 1794.

The Plymouth siblings as Amor and Psyche, by Angelica Kauffmann, 1795.

Philipp IV of Spain with his sister Infanta Anna, by Bartolomé González y Serrano, 1612.

Het gestoorde pianospel (Interrupted Piano Practice), by Willem Bartel van der Kooi, 1813.

Anders and Margaret Eek, by Knut Ekwall, before 1912.

Two boys said to be children of the Poulett family of Hinton St. George, English School, 1600.

 Siblings, by José Cruz Herrera, 1928.

Unknown, unknown artist, 1874.

The Shelly Children, John and Charlotte Anne, by William Beechey, circa 1771-1772.

British school, circa 1800-20.

Four unknown siblings with their dog, attributed to Salomon Mesdach, 1627.

Auguste, Eugénie, and Marie of Württenberg, the daughters of Count Wilhelm of Württenberg, later Duke of Urach, by his first wife, unknown artist, circa 1852.

Sisters, by Wilfred Gabriel De Glehn, 1927.

Portrait of Leana and the Lambeth children, unidentified artist active in New Orleans, circa 1848.

The portrait of the boy is believed to be posthumous.

Edward and Sarah Rutter, by Joshua Johnson, 1805.

Siblings, by Louis Appian, 1893.

Harold and Eric Molson, sons of J. Elsdale Molson, by Gerald Fenwick Metcalfe, circa 1902.

Siblings, by Olga Boznańska, 1898.

Charles and Henry Brandon, sons of Charles Brandon, Duke of Suffolk, and Catherine Willoughby, by Hans Holbein the Younger, 1541.

Four siblings, unknown artist, circa 1840.

Brothers, by Lydia Field Emmet, 1909.

Brothers Hendrick, Johannes, and Simon, by Thomas de Keyser, circa 1627-32.

Two sisters, by Carl Ludwig Johann Christinek, 1772.

The children of Captain R. D. Pritchard, by Philip August Gaugain, 1827.

Geschwister im Schlosspark (Siblings in the Castle Park), by Heinrich Lauenstein, 1888.

Half ladies and three-quarter females - a selection of portraits

 

Gräfin Chorinsky née Prinzessin Esterházy, by Friedrich von Amerling, 1838.

Elizabeth Howard, by Anthony van Dyck, circa 1638-40.

Catherine Clemens, by George Romney, 1788.

Unfinished portrait of Louise Élisabeth d'Orléans, by Jean Ranc, 1724.

Marie Rinteau, called mademoiselle de Verrières, by François-Hubert Drouais, 1761 and later.
Apparently exhibited by Drouais at the Salon of 1761, the sitter's hairstyle was altered by the artist or someone else during the 1770s to make it more en vogue.

Susanna Highmore, the artist's daughter, by Joseph Highmore, circa 1740-45.

 Smiling Girl / Courtesan Holding an Obscene Image, by Gerrit van Honthorst, 1625. 
The model holds an image of a naked woman, perhaps the model herself, covering her face and captioned in Dutch Wie kent mijn naers van Afteren (Who
can recognize my backside from behind?). The small image likely advertises her services, as this was a common practice in the sex industry of the time.

Archduchess Maria Magdalena, Grand Duches of Tuscany, by Frans Pourbus the Younger, circa 1603-04.

Portrait of a Lady at a Harp, by Johann Julius Heinsius, 1785.

Madame Alexandre Dumas fils née Nadezhda von Knorring, by Jules Lefebvre, circa after 1864.

Madame Louis-François Bertin de Veaux née Augustine Bocquet, by Anne-Louis Girodet-Trioson, 1809.

Portrait of a Lady in a Green Dress, by Bartolomeo Veneto, 1530.

Mrs. John Sargent née Charlotte Bettesworth, by George Romney, circa 1778.

Giovanna Bagnara, by Pierre Subleyras, circa 1739.

Girl with a Canary, by Novak Radonić, 1865 (or 1858, depending on the source).

Olive Craster, by Nathaniel Dance-Holland, 1762.

Mary Josephine Drummond, condesa de Castelblanco, as Astrea, by Jean-Baptiste Oudry after Nicolas de Largillière, circa 1716.

Mrs. John Dawes, by George Romney, circa 1765-69.

Victoria, Duchess of Kent (the mother of Queen Victoria), by Franz Xaver Winterhalter, 1849.