If you purchased a home, you can deduct certain closing costs in the year of the purchase. You can also deduct mortgage interest and property taxes each year that you pay them.
The $100K from your family, whether it was a cash gift or a gift of equity, is not taxable to you at any point.
When you sell the house, your capital gain, if you have one, is the difference between the selling price and the purchase price. The size of the loan does not matter. If you sell for more than you paid, you have a gain. If the home had been your personal residence at the time of the sale, the first $250,000 of gains is tax free, or $500,000 if you are married. So you could potentially walk away with $1 million and not owe tax. But that all happens when you sell.
