Are the siblings and father of someone that is getting married on lag baomer night allowed to take haircuts on lab baomer?
Mazal Tov! They should be zoche to biuld a bayis neeman b’yisroel, and you should have muich nachas from them.
Regarding your question, if I understand it correctly, the chasuna is on the night after lag baomer. Therefore according to the Rema we can all take haircuts after daybreak of lag baomer, and not just the family of the chosson. If you usually keep a different sefira and don’t finish on Lag Baomer, it is possible to switch the time period that you don’t take a haircut, and if you kept sefira from Pesach you can also take a haircut. If you really have to, it would also be permitted to take the haircut on the eve of lag baomer, but it is better not to.
O:CH 493-2, M:B 11, Shut Chasam Sofer 142, MInchas Yitzchok 4-84, Kovetz Halachos (Sefira) 8-2.