I gifted my sister $100 worth of ETH last year when it was hovering around $2k. She also likes to invest on Bitcoin but wants to go all-in. But I told her not to because its very risky if she will convert all of her savings into Bitcoin.
My friend you are underestimating Bitcoins according to me. Your sister might have done a proper research before thinking of going all in with Bitcoins. Bitcoins have the highest marketcap compare to any other coins, and its limited in numbers, so definitely as the time passes it will become more valuable, and hence due to limited in numbers the price will go high. I guess your sister was thinking right, Bitcoins are always best for long term holding.