If you feel uncomfortable, you should have a heart to heart talk with your boyfriend about it because it's normal for us to want your space and privacy.

Six people in a two bedroom house is too many. Nine may be a health hazard.

If you are renting, having that many people may get you evicted. The fair housing guidelines in the U.S. generally state maximum occupancy as 2 people per bedroom plus 1. This can be stretched to 3 per bedroom if the rooms are really big. So your maximum would only be 5-7 people. It may seem mean to tell family that they need to get the hell out as soon as possible, but if they stick around long enough for your landlord to figure it out - you could all end up homeless.

why so much kids?

Have a backbone and stand up for yourself. There's no reason your boyfriend's brother and his kids should be staying in the house, unless if they have no place to go etc in which they should probably get a hotel before they have 4 people sleeping on the floor... 9 people is clearly way too many. The max should be 4 or 5..

I get that this is obviously for financial reasons if this is intended to be long term, or maybe safety reasons if the cause is short term and they'll go back home shortly, but every person at or above the age of majority living there could get in some major legal trouble for that. Unless the living room is huge, your house is probably rated to hold 3-5 people, and your either nearly double or over it. If there are kids involved, you could even get charged with reckless endangerment of a minor, depending on laws where you live. That's excluding some of the more nit picky laws about housing, particularly in regards to children. If it isn't just a visit, then at least one family unit needs to apply for HUD housing, possibly more. Because eventually someone will let it slip that the house is dangerously over full, and then everyone could wind up in a kettle of hot water.