Is it forbidden to celebrate the birthday of a son, or his success, or the end of his military service – with only his mother and sister present?

The ruling on celebrating birthdays has been previously clarified, and the small number of celebrants does not change this ruling.

As for celebrating success or completing a service, there is no harm in it, provided that it is free from intentions of pride and boastfulness, and that it does not involve extravagance or wastefulness.

Evidence for this includes that when Umar ibn al-Khattab (may Allah be pleased with him) completed Surah al-Baqarah, he slaughtered a camel. And Ibn Hajar held a grand feast upon completing his composition of "Fath al-Bari." Likewise, Siddiq Hassan Khan held a feast for his tafsir "Fath al-Bayan."