Since you can't get a straight answer from dad, I'd suggest that you talk to your sisters to see what they know. Don't be petty or confrontational. Don't be jealous. Just ask and see what they say.
Based on the limited information you have given us, I'll make the following guesses.
-Your sisters are older than you.
-Your grandma gave them property directly because they were adults.
-Because you were a minor (or you were a minor when she drew up her will), she put liquid assets (stocks, cash, bonds) into a trust for you and made your father the trustee (manager) of the trust.
-Over the years, the trustee (dad) spent the money on himself. Now, there's next to nothing left.
If my guess about the trust is correct, then what your dad did is illegal, but you have no real recourse. The law says you could sue him. You might not win, but even if you did win, he probably has very little assets other than his home. You wouldn't get the home. He could just declare bankruptcy and you'd get nothing. Plus, you'd be out some hefty attorney fees.
Again, I'd start by asking the sisters. Remember, they aren't the problem. The decisions were made by grandma and dad. Your sisters owe you NOTHING.
And by the way, you're making the assumption that there is a trust. There may never have been a trust. Your grandma may have just willed the money to dad with the understanding that he was to give it to you when he died. Sorry, but that is NOT binding.
Again, you should assume you will get nothing. Accept that. Then, ask your sisters if they know what happened. Regardless of what they say, the outcome for you is the same. You get nothing. Just accept it and move on.