Can I sue myuncle for my dad’s ashes/belongings?

Could you sue?  Sure.  Would you be successful, or would suing your uncle be worth the time and effort?  Probably not, unfortunately.

First, I realize that you're limited by space on this forum, but you don't indicate at all if your father left a Will or not.  That fact could be pretty important, at least as it relates to the personal effects that belonged to your father.

Second, depending on how old you are now, you're talking about something that your uncle may have done wrong several years ago.  If there is any cause of action that you could pursue, it's pretty likely to have run by now -- even though you were a minor when your father died.

Last, even if you had a surviving cause of action, you should consider what it might cost you in order to achieve your picture of justice.  Litigation expenses are not always borne by the loser, and litigation itself is rarely, if ever, inexpensive.  If I told you that it might cost $100 to correct what you believe is wrong, you'd probably be on board.  If I told you that the costs of litigation could easily climb to $10,000, $20,000 or $30,000, you might take a step back.

I'd spend some serious time thinking about how important it is to you to have more of your father's ashes and personal belongings than you do now.  If it's that big of a deal, and if the warning of significant time and expense involved doesn't dissuade you, then you need to sit down with a probate litigation attorney near you to discuss the many details that you just don't have the space to get into using this forum.  Good luck!