At my wits end with sleeping issues!

My DD is now 21 mos old and ever since my DS was born 9 wks ago, her sleeping routine is non-existent. As a matter of fact, her 9wk old brother now sleeps better than she does!

Every night it's a battle to get her into bed. She cries for at least 1 - 1.5 every night. Then when she finally does fall asleep, we can count on her waking up at least 1x during the night crying. We've tried everything and feel like big fat failures as parents because we just can't get our little 'angel' to sleep!

Any advice is welcome...I'm so tired and depressed over this.

TIA