Am I crazy or is something wrong?

So my fiancé and I have been together for almost 3 years and engaged for 2 months. He has always drank at night but there were days were he wouldn’t, but for the last month he has been drinking non stop starting at like 6pm or 7pm. He says he does have a problem and gets mad everytime I bring it up. But he drinks to the point of being drunk and I don’t like it. He even does it when his son comes over to spend the night. Also he keeps talking to my sister. They talk about her problems like her wanting to lose her virginity, and guys she likes, and he even told her that we are going to buy her a phone for her birthday. But he hasn’t told me about it. He never even got me anything for my birthday. I tell him talking to my sister bothers me but again he gets upset everytime I bring it up. She doesn’t even know why it bothers me. Also I have 2 other sisters that he has never talked to. She’s also a teenager. Why is he talking to a teenager. He says he doesn’t finder her sexually attractive and they don’t talk about anything bad, but he waits to text her when I’m not around. It makes me feel like he’s cheating on me with my sister. I just feel like with the drinking and this, I don’t know if it’s something I want to put up with for the rest of my life and it doesn’t seem like he will give those 2 things up. Is it a good reason to leave a 3 year relationship?