His brother may really just want one last trip together before your BF settles down and will not be able to go with him.

I have an older sister who is my best friend. We are by no means young and we both have extended familes. I wanted to take a trip with just my sister. She agreed and we picked a date. I found a resort and looked into travel times, etc. Then, my sister announced she had invited her 30 year old daughter and her 1 year old grandson! I love my niece and nephew dearly but I do not want to spend my week being an "extra" on their family vacation. I want to spend time with my sister. I cancelled the trip.

Give your future BIL a chance. You may be taking this too personally.