Blended Families - How hard is it suppose to be?

[LeonieP]

I've been married now a few years an my wonderful husband has 3 children & I have 1. They are aged between 12-17. My problem is that at first everyone got along very well - it was great! Then the first of the terrible girl teens started which lead to arguements in the home. I'm going to that party & your buying me booze... No we arn't .. yes you are scenario. It didn't matter what the subject was about 'it turned nasty!! When one of the girls was 14 she started seeing a boy. Then wanting him to sleep over cause MUM lets me.. We looked further into it an yes their mother was allowing boys to sleep over an in the same bed (even her friends were allowed their bf's over for the night) This is where it started to get very nasty as we have rules in the home an won't bend over just because their mum does. So the kids basically stopped coming over week about. We've since found out their mums new bf is into Drugs (not just the smoking type) The 14yr old was on ICE their for awhile (after she stopped coming to our place) but we believe she's off it now.
Troubles are though cause now they are constantly allowed to do as they please I get used as a wipping board. By their mother an the kids.. they are constantly just ringing me up an abusing me 'F witt' bla bla.. We pay Child Support $1300 per month yet the mother won't give them any money an we are constantly handing out extra. I keep trying to tell my husband we can't keep giving in although being male he'd rather do it than have an arguement.
How can I try to resolve these issues before my husband an I lose each other?