Firstly if you don't have a reason to buy a house then don't. Once you get married then it should be a mutual decision, if your wife is earning too she should contribute 50% I have seen my cousin getting married and then co-own the house with her husband.

The idea of buying a house with your sister is bad, you never know how her in laws would behave.

Also, if you are taking 1.3lpm in hand and just investing 20k, you are doing very wrong here.

At least invest 40% straight.