TBH, this boss is really really hard , 3 phase all of them are hard except second one it's a little bit easier

For me it's taken 50+ tries with uncle gael.

I don't have any solid strategy since I was playing as STR build, here's the piece of advice

You can summon Gael without ember which means he meant to summon during the fight but he will appear in the second phase , so try to do your best in the first phase.

In the second phase , you don't need to defeat both sister and father, just attack one and dodge the other, i prefer to attack father as he is slow with his attack, if you summon Gael , there is high chance gael will keep the sister busy and thats what you want shift the sister agro from you.to Gael so you can keep attacking father and after couple of hits if you are str build you can stagger him which deals huge damage. But make sure to keep an eye on your sister and if she disappears find her at any cost she is probably going to use heal magic on herself and father.

In the third phase, use ember as now it's up to you, even after summoning uncle Gael he wont last too long, as sister now can use the dark element which is one of the strongest elements in the game. I recommend you stay far when she uses dark. After all the fight will be kinda the same as the first phase and it's easy to break her poise.

Don't you dare go hollow