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.
It's not that he is underestimating btc but he is only trying to be careful and he is right that we shouldn't all in our savings because btc value is volatile and that is our savings man. It supposed to be separated and not to be used in investing because it will be use in the future or for emergency purposes. It is more stable than btc.
Another thing on why doing all in is bad is because the price can possibly go down. It enables us to invest again for a better gain. Eth has an unknown supply before but now that it migrated in POS, its supply should be known now and it was now limited. This makes eth a good addition aside from our btc, if we are hodling for long term.