I know of his uncle Hamza ibn Abdul Mutalib, and his cousin Abu Sufyan ibn Al-Harith but I would like a complete list.
The foster siblings of the prophet () were:
Hamzah ibn 'abd al-Muttalib حمزة بن عبد المطلب () as we have an evidence in the sunnah saying:
The Prophet (ﷺ) said about Hamza's daughter, "I am not legally permitted to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother." (Sahih al-Bukhari)
Before the prophet () went to Halimah as-Sa'adia he also was breast fed by Thuwaiba who was a servant of abu Lahab. Thuwaiba has breast fed Hamzah () first, then Muhammad() before breast feeding abu Salamah ().
Abu Salamah أبو سلمة () as we have an evidence in the sunnah saying:
I said, "O Allah's Messenger (ﷺ)! Marry my sister, the daughter of Abu Sufyan." He said, "Do you like that?" I said, "Yes, for even now I am not your only wife; and the most beloved person to share the good with me is my sister." The Prophet (ﷺ) said, "But that is not lawful for me (i.e., to be married to two sisters at a time.)" I said, "O Allah's Messenger (ﷺ)! By Allah, we have heard that you want to marry Durra, the daughter of Abu Salama." He said, "You mean the daughter of Um Salama?" I said, "Yes." He said, "By Allah ! Even if she were not my stepdaughter, she would not be lawful for me to marry, for she is my foster niece, for Thuwaiba has suckled me and Abu Salama; so you should neither present your daughters, nor your sisters to me." (See for example in Sahih al-Bukhari)
Abu Salamah is Abdullah ibn al-Assad ibn Hilal ibn 'Abdullah ibn 'amr ibn Makhzum al-Makhzumi عَبْدُ اللَّهِ بْنُ عَبْدِ الأَسَدِ بْنِ هِلالِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ بْنِ مَخْزُومٍ الْمَخْزُومِيُّ he is also the son of his aunt Barrah bint 'Abdulmttalib.
3-5. And he () has furthermore foster siblings from Halimah as-Sa'adiya who are:
- a-Shayma' bint al-Harith ibn 'Abd al-'Uzza ibn Rafi'a الشَّيْمَاء بِنْت الْحَارِث بْن عَبْد الْعُزَّى بْن رِفَاعَة who is also known as الشَّمَا a-Shama (without ya') or خِذَامَة Khidamah, she was also called Jidamah جِدَامَة or Hudafah حُذَافَة.
- 'Abdullah ibn al-Harith عَبْد اللَّه بْن الْحَارِث.
- Anisah bint al-Harith أُنَيْسَة بِنْت الْحَارِث.
Ibn Kathir in his al-Bidaya wa an-Nihaya according to this fatwa -in Arabic- on islamweb #148107 quoted ibn Ishaaq counting his () foster siblings: In the following I will translate from Arabic, this is my own translation take it carefully
وإخوته عليه الصلاة والسلام يعني من الرضاع: عبدالله بن الحارث، وأنيسة بنت الحارث، وحذافة بنت الحارث وهي الشيماء.
And his sibling peace be upon him -meaning form fostering- are: 'Abdullah ibn al-Harith, Anisah bint al-Harith and Hudafa bint al-Harith who is a-Shayma'.
- Ibn al-Qayyim according to the same fatwa added in his zaad al-Ma'ad زاد المعاد that Halimah as-Sa'adiya also breast fed his cousin abu Sufyan al-Harith ibn 'Abd al-Muttalib أبو سفيان بن الحارث بن عبدالمطلب
By this we can count the following foster siblings:
- Males: Hamzah, Abu Salamah (foster mother of both: Thuwaiba), 'Abdullah ibn al-Harith and Abu Sufyan ibn al-Harith ibn 'Abd al-Muttalib (foster mother of both: Halimah).
- Femnales: Anisah bint al-Harith and a-Shayma' (or Hudafah etc.) bint al-Harith both are daughters of Halimah.
Here another source -in Arabic- that I found helpful.
This website of 'Amr Khalid the Egyptian "preacher" adds 'Uthman ibn Madh'on عثمان بن مظعون () -a sahabi who died 3 a.H.- to this list, but I couldn't find any backup for this. The same site also quotes Hafs the son of Halimah as-Sa'adya حفص بن حليمة السعدية who is counted as sahabi in al-Isabah الإصابة في تمييز الصحابة of ibn Hajar al-'Asqalani, but as most historians only count three children of Halimah it is possible that Hafs and 'Abduallah are the same person and Allah knows best!