M.B.
My oldest was 15 when the youngest was 1yr and we went thru the same thing. Elder would be walking to her room, shut the door and little one would be following and screaming, broken-hearted.
First thing we did was decided that older sis should never pass her by without at least acknowledging her (hi, a kiss, whatever).
Second, big sis needed her space... my respect for that went a long way.
Third, big sis would spend blocks of time with her.... then when she was done... hand baby off to me. This technique really helped because it "trained" baby, so when big sis didnt have time to spend... running in and out as teenagers do, she could give her a quick kiss and hug, hand her off to me.. and all was good.