My husband has a very close relationship with all three of his sisters. He was the youngest, the pampered only son (I like to say that he was the "little raja") and his sisters still compete to show him affection. As his wife, I've been warmly welcomed into the brother-sister relationship and the celebration of the brother-sister holiday Bhai Dooj (just after Diwali) is one demonstration of that.
Bhai Dooj is celebrated in North India by brothers and sisters. Brothers give their sisters a gift (usually cash) and sisters perform aarti to their brothers and give them a gift of food or sweets. This demonstrates the bond of love between the siblings both now and into the future.
This was the first Bhai Dooj they've spent together in years, and the first since we got married. This time I was included in the ceremony too. Each sister performed aarti, put tilak (red paste) and rice grains on our foreheads, and gave us a gift. It felt really special to experience the ceremony with my husband and all three didis. Afterward, my little niece performed the ceremony with her cousins (honorary brothers).
Here is my youngest SIL putting tilak on my head as part of the ceremony:
If you go out in public on that day, you'll see lots of men and boys walking around with tilak on their foreheads, a symbol of the devotion their sisters have for them!