You could try signalling that unless they trade with you you'll put them at a disadvantage. Consider - "Player 1, both you and Player 2 want this property. This property for one of your properties is a fair swap that benefits both of us, but if you turn me down I'll trade it to Player 2 for their best offer, which benefits Player 2 a lot and me only a little. Player 2, if you don't give me even the small amount I ask for, I'll randomly give it to some other player." If you signal credibly (ie you actually do it if someone calls your bluff) then Player 1 should make the trade provided he values your property more than you knocking yourself out of the game (ie the trade really DOES have to be a fair deal - you can't just use this to up your bargaining ante).

Part of your argument could be the (truthful) observation that the losers in a trade aren't the people who made the less valuable trade, but the people who didn't trade at all - if you play at a very conservative table it might be in your interest to trade at a disadvantage and exploit the increased variance a monopoly gives you.

The major downside here is that many people don't play Monopoly to win, and strategies that optimise for winning using game theory and the like are seen as 'unsporting'. Sometimes you just need to accept that to keep your family happy you have to play Monopoly and get bored.

Talk to one family member ahead of time, get them to trade with you for mutual benefit, and see how long it takes the rest of your family to notice that the two of you are winning more?