"I'm on the hunt for info about capital gains tax, deed transferaship and whatever other fees might affect us in the future. If it's less than 50K then it would be worth it"
I think you're being blinded here and not actually considering what is the major issue
You and your spouse/partner cannot get a mortgage in your own name at the rate you want. So, you/your sister seem prepared to go along with a scenario where your sister ( for the purposes of this situation now called Elaine) buys a house, in her sole name, and takes out a mortgage on the property in her sole name. You and your spouse/partner will have no legal interest in this property - it will be Elaine's. Elaine is being generous, in one way, but also very foolish - she is taking all the risk and , should anything go awry in the future, she is the one with the mortgage liability.
You talk about "capital gains tax, deed transferaship and whatever other fees might affect us in the future" but without being specific as to what you all envisage happening (1) now and (2) down the line.
Reading between the lines, it sounds ( and maybe you could be more specific) as if you anticipate that Elaine will ultimately fade out, she will transfer the property to you and you will take out a new mortgage? or maybe you anticipate "transferring" her mortgage also? (Not possible).
You are looking at CAT - gift tax for you, CGT for Elaine ( if the property increases in value), stamp duty on a Transfer Deed
I'm going to suggest that you run your proposal past a solicitor who will be able to tease out what you want, what is possible/not possible, and what the pitfalls will be.
I'm with Montbretia on this - better to own the house in your own names, with your own mortgage.
mf